Blockstream is a blockchain technology company co-founded by Adam Back, Gregory Maxwell, Austin Hill and others, and led by Adam Back and Gregory Maxwell. Blockstream is one of a number of institutions that provide funding for the development of Bitcoin Core, the predominant network client software.
The company is focused on developing bitcoin applications: specifically sidechains, as well as other applications.  Blockstream has raised $76M to date from investors including Horizons Ventures and Mosaic Ventures.   Blockstream employs several prominent bitcoin developers, including Adam Back (President, Blockstream), Gregory Maxwell (CTO, Blockstream), Mark Friedenbach (Co-founder, Blockstream), Pieter Wuille (Bitcoin Core developer), Samson Mow (CSO), and Christopher Allen (co-author of IETF Transport Layer Security).    Blockstream is one of the largest contributors of funding for Bitcoin Core. 
On October 12, 2015, Blockstream announced the release of its Liquid sidechain which could allow for the transfer of assets between the sidechain and the main blockchain.    Blockstream produces software that facilitates interoperability between the main chain and the sidechain.  Blockstream claims that Liquid reduces the delays and friction involved in a normal transfer of bitcoin. Blockstream asserts participating exchanges–Bitfinex, BTCC, Kraken, Unocoin and Xapo can make near instant exchanges between their accounts and orderbooks.   The company has proposed that the Liquid sidechain, which is a pegged sidechain, be added to the bitcoin protocol.    The source code for sidechains has been released on an open source basis. 
In 2017 Blockstream announced a plan to offer one way satellite broadcasting of the full bitcoin blockchain.  The network as of 2017 is only a one way network and the user still needs an internet connection to send transaction.  The company plans to earn fees for value added services on the network, such as creating an API to allow companies to send additional data over its network. 
In addition to its corporate initiatives, blockstream is also involved in a number of community steering and open source programs.
Blockstream hired Rusty Russell, a well-known developer famous for his work on Linux kernel, to develop an implementation of the Bitcoin Lightning Network (LN). Russell has a four-part LN explanation on his blog.  The Lightning network proposes to reduce transaction costs by allowing nodes to hold some transaction data in the cache before submitting it to the chain.
In May 2016, Blockchain.info announced Thunder, which claims to be an implementation of Lightning.
In May 2017, Segregated Witness activated on Litecoin, enabling the Lightning Network to be used. In May 2017 Christian Decker of Blockstream sent the first microtransaction using lightning on Litecoin.
Blockstream released an open source codebase and testing environment for its sidechains product to the public under an open source license. The Elements Project is a community effort to create and test new extensions to the Bitcoin protocol, whose early contributions include Confidential Transactions by Gregory Maxwell, and Segregated Witness by Pieter Wuille.
In April 2017, Blockstream released a paper on Confidential Assets, an extension of Confidential Transactions (which is itself derived from an Adam Back proposal for homomorphic values applied to Bitcoin).
Among other contributions:
We begin by describing an efficient rangeproof for Pedersen commitments over the interval [0, m n − 1], which has total size proportional to 1 + nm, using a variant of a folklore bit-decomposition based rangeproof, in which numbers are expressed in base m and each digit is proven to lie in [0, m − 1] using a ring signature.
The rangeproofs are implemented in.
Blockstream participates in Hyperledger as a general member. The Hyperledger project is a cross-industry collaborative effort to create blockchain based open standard for distributed ledgers for globally conducted business transactions.