ICQ is an instant messaging client that was first developed and popularized by the Israeli company Mirabilis in 1996. The name ICQ derives from the English phrase "I Seek You".  Ownership of ICQ passed from Mirabilis to AOL in 1998, and from AOL to Mail.Ru Group in 2010.
The ICQ client application and service were initially released in November 1996 and the client was freely available to download. Users could register an account and would be assigned a number, like a phone number, for others to be able to contact them (users could also provide handles). ICQ was the first stand-alone instant messenger and the first online instant messenger service — while real-time chat was not in itself new to the internet ( IRC being the most common platform at the time), the concept of a fully centralized service with individual user accounts focused on one-on-one conversations set the blueprint for later instant messaging services like AIM , and its influence is seen in modern social media applications.
At its peak around 2001, ICQ had more than 100 million accounts registered.  At the time of the Mail.Ru acquisition in 2010, there were around 42 million daily users.  Since 2013, ICQ has 11 million monthly users. 
- ICQ features include offline user messaging, multi-user chats, free daily-limited SMS sending, resumable file transfers , greeting cards, multiplayer games and a searchable user directory. Users can use emoticons while chatting with other users.
- ICQ6 was launched on April 17, 2007, and offered a single communication platform that combines the various user options: instant messaging services, free SMS from ICQ to mobile , voice and video communication. The software's sounds were created by the Israeli psychedelic trance duo Infected Mushroom . Among the new additional features in ICQ6 are Quick IM, which allows users to send a short message without opening a conversation window, a "follow me" service directly to the user’s mobile, a multi-chat service and support for Zlango , the animated icons language.
ICQ provides all users additional services and content products:
- ICQ TV—An online video magazine. The magazine broadcasts content for teens 24/7.
- ICQ SIM Card (together with United Mobile )—A SIM card that enables users traveling across Europe to use their cellular telephones while paying a discount price.
- ICQ Game Center—A games platform that enables the user to play with and/or against other users. Available by clicking a button in the contact list.
- ICQ2Go—A web instant messaging option for users who cannot download the program onto their computers or use it on the networks they are accessing (for various reasons, like security restrictions or firewalls ).
- ICQ for Mac  (Beta)—Released in February 2010, a version of the client that was developed using the Adobe AIR platform , making it usable on additional operating systems which support the AIR runtime, such as Linux .
Also, users can choose and select their own avatars for their profile pages. In this way, they can protect their privacy.
ICQ users are identified and distinguished from one another by UIN, or User Identification Numbers, distributed in sequential order. The UIN was invented by Mirabilis , as the user name assigned to each user upon registration. Issued UINs started at '10,000' (5 digits) and every user receives a UIN when first registering with ICQ. As of ICQ6 users are also able to log in using the specific e-mail address they associated with their UIN during the initial registration process. Unlike other instant messaging software or web applications , on ICQ the only permanent user info is the UIN, although it is possible to search for other users using their associated e-mail address or any other detail they have made public by updating it in their account's public profile. In addition the user can change all of his or her personal information, including screen name and e-mail address, without having to re-register. Since 2000 ICQ and AIM users are able to add each other to their contact list without the need for any external clients. As a response to UIN theft or sale of attractive UINs, ICQ started to store email addresses previously associated with a UIN.  As such UINs that are stolen can sometimes be reclaimed. This applies only if (since 1999 onwards) a valid primary email address was entered into the user profile.
The founding company of ICQ, Mirabilis, was established in June 1996 by five Israelis: Yair Goldfinger, Sefi Vigiser, Amnon Amir, Arik Vardi, and Arik's father Yossi Vardi .  They recognized that many people were accessing the internet through non- UNIX operating systems , such as Microsoft Windows , and those users were unfamiliar with established chat technologies, e.g. IRC .
The technology Mirabilis developed for ICQ was distributed free of charge. The technology's success encouraged AOL to acquire Mirabilis on June 8, 1998, for $287 million up front and $120 million in additional payments over three years based on performance levels.  At the time this was the highest price ever paid to purchase an Israeli technology company. In 2002 AOL successfully patented the technology.  
After the purchase the product was initially managed by Ariel Yarnitsky and Avi Shechter. ICQ's management changed at the end of 2003. Under the leadership of the new CEO, Orey Gilliam,  who also assumed the responsibility for all of AOL's messaging business in 2007, ICQ resumed its growth; it was not only a highly profitable company, but one of AOL's most successful businesses. Eliav Moshe replaced Gilliam in 2009 and became ICQ's managing director. 
In April 2010, AOL sold ICQ to Digital Sky Technologies, headed by Alisher Usmanov , for $187.5 million.  While ICQ was displaced by AOL Instant Messenger, Google Talk, and other competitors in the U.S. and many other countries over the 2000s, it remained the most popular instant messaging network in Russian-speaking countries, and an important part of online culture.  Popular UINs demanded over 11,000 rubles in 2010. 
In September of that year, Digital Sky Technologies changed its name to Mail.Ru Group. Since the acquisition, Mail.ru has invested in turning ICQ from a desktop client to a mobile messaging system. As of 2013, around half of ICQ’s users were using its mobile apps, and in 2014, the number of users began growing for the first time since the purchase. 
- ICQ 99a/b the first releases that were widely available.
- ICQ 2000 incorporated into Notes and Reminder features.
- ICQ 2001 included server-side storage of the contact list, allowing for its synchronization on multiple computers.
- On December 19, 2002, AOL Time Warner announced that ICQ had been issued a United States patent for instant messaging.
- ICQ 2002 was the last completely advertising-free ICQ version.
- ICQ Pro 2003b was the first ICQ version to use the ICQ protocol version 10. However, ICQ 5 and 5.1 use version 9 of the protocol. ICQ 2002 and 2003a used version 8 of the ICQ protocol. Earlier versions ( ICQ 2001b and all ICQ clients before it) used ICQ protocol version 7.
- ICQ 4 and later ICQ 5 (released on Monday, February 7, 2005), were upgrades on ICQ Lite. One addition was Xtraz, which offers games and features intended to appeal to younger users of the Internet. ICQ Lite was originally an idea to offer the lighter users of instant messaging an alternative client which was a smaller download and less resource-hungry for relatively slow computers.
- ICQ 5 introduced skins support. There are few official skins available for the current ICQ 5.1 at the official website; however, a number of user-generated skins have been made available for download.
- ICQ 6 , released on April 17, 2007, was the first major update since ICQ 4. The user interface has been redesigned using Boxely , the same rendering engine used in AIM Triton . This change adds new features such as the ability to send IMs directly from the client's contact list. ICQ has recently started forcing users of v5.1 to upgrade to version 6 (and XP). Those who do not upgrade will find their older version of ICQ does not start up. Although the upgrade to version 6 should be seen as a positive thing, some users may find that useful features such as sending multiple files at one time is no longer supported in the new version. At the beginning of July 2008, a network upgrade forced users to stop using ICQ 5.1 - applications that identified themselves as ICQ 5, such as Pidgin , were forced to identify themselves as ICQ 6. There seems to be no alternative for users other than using a different IM program or patching ICQ 5.1 with a special application.
- ICQ 7.0 , released on January 18, 2010. This update includes integration with Facebook and other websites. It also allows custom personal status similar to Windows Live Messenger ( MSN Messenger ). ICQ 7.0 does not support traditional Chinese on standard installation or with the addition of an official language pack. This has made its adoption difficult with the established user base from Hong Kong and Taiwan where traditional Chinese is the official language.
- ICQ 8 , released on February 5, 2012 - "Meet the new generation of ICQ, Enjoy free video calls, messages and SMS, social networks support and more."
- ICQ 10.0 10242 , released January 18, 2016 - Newest update. 
Privacy and copyright
Please be aware that any information you may have published in a public area (Public Area) may be accessible to ICQ or third parties and used by ICQ or such third parties. Therefore carefully consider whether you wish such information to be accessible. Public areas may include your user profile (depending on the level of your privacy settings) any blogs, message boards, forums or similar services made available through the ICQ Services.
ICQ accounts may be deleted by the user.
Pressure on alternative clients
AOL pursued an aggressive policy regarding alternative ("unauthorized") ICQ clients.
- In July 2008 changes were implemented on ICQ servers causing many unofficial clients to stop working. These users received an official notification from "ICQ System".
- On December 9, 2008, another change to the ICQ servers was made. The clients that were sending Client IDs not matching ICQ 5.1 or higher stopped working.
- On December 29, 2008, ICQ press service distributed a statement characterizing alternative clients as dangerous. 
- On January 21, 2009, ICQ servers started blocking all unofficial clients in Russia and Commonwealth of Independent States countries.  Users in Russia and Ukraine received a message from UIN 1:
- ICQ не поддерживает используемую вами версию. Скачайте бесплатную авторизованную версию ICQ с официального web-сайта ICQ.
- The version you are using is not supported by ICQ. Download a free authorized ICQ version from ICQ’s official website."
On icq.com there is an "important message" for Russian-speaking ICQ users: "ICQ осуществляет поддержку только авторизированных версий программ: ICQ Lite и ICQ 6.5." ("ICQ supports only authorized versions of programs: ICQ Lite and ICQ 6.5.")
- On February 3, 2009, the events of January 21 have repeated.
AOL 's OSCAR network protocol used by ICQ is proprietary and using a third party client is a violation of , nevertheless a number of third-party clients have been created by using reverse-engineering and protocol descriptions. These clients include:
- Adium : supports ICQ, Yahoo! , AIM , MSN , Google Talk , XMPP , and others, for macOS
- Ayttm : supports ICQ, Yahoo!, AIM, MSN, IRC , and XMPP
- bitlbee : IRC gateway, supports ICQ, Yahoo!, AIM, MSN, Google Talk, and XMPP
- centericq : supports ICQ, Yahoo!, AIM, MSN, IRC and XMPP, text-based
- climm (formerly mICQ): text-based
- Fire : supports ICQ, Yahoo!, AIM, MSN, IRC, and XMPP, for macOS
- Jimm : supports ICQ, for Java ME mobile devices
- Kopete : supports AIM, ICQ, MSN, Yahoo, XMPP, Google Talk, IRC, Gadu-Gadu, Novell GroupWise Messenger and others, for Unix-like
- Meetro : IM and social networking combined with location; supports AIM, ICQ, MSN, Yahoo!
- Miranda IM : supports ICQ, Yahoo!, AIM, MSN, IRC, Google Talk, XMPP, Gadu-Gadu, BNet and others, for Windows
- Naim : ncurses -based
- Pidgin (formerly Gaim): supports ICQ, Yahoo!, AIM, Gtalk, MSN, IRC, XMPP, Gadu-Gadu, SILC, Meanwhile, ( IBM Lotus Sametime ) and others
- QIP : supports ICQ, AIM, XMPP and XIMSS
- stICQ : supports ICQ, for Symbian OS
- Trillian : supports ICQ, Yahoo!, AIM, MSN, IRC, Google Talk, XMPP and others
AOL supported clients include: