Area codes 410, 443, and 667 are telephone area codes serving the eastern half of the U.S. state of Maryland, including the Baltimore metropolitan area and the Eastern Shore. The 410 area code is the main area code, while the 443 and 667 codes are overlay codes. 443 and 667 were primarily used with cell phones and CLEC carriers such as Comcast or Cavalier Telephone when introduced but have since become universal in their carrier availability.

Before these area codes were created, all of Maryland had been served by 301 since the institution of area codes in 1947, even though the state is home to two very large metropolitan areas—Baltimore and the Maryland suburbs of Washington, D.C. (area code 202). This made Maryland one of the most-populous states to be served by a single area code. However, by the late 1980s, 301 was on the verge of exhaustion due to the rapid growth of the Baltimore and Washington suburbs, as well as the proliferation of fax machines and pagers.

The number shortage problem was exacerbated by the use of 202 as a de facto overlay for the entire Washington metro area. Every number assigned on the Maryland side of the metro, as well as in northern Virginia's area code 703, was given a "hidden" number in 202, making it possible to complete a call within the metro area with only seven digits. One consequence of this was that a central office prefix could only be duplicated in areas considered to be at a safe distance from the Washington area. Thus, if a 202-574 number was in use in the District or a 703-574 number was in use in northern Virginia, the corresponding 301-574 number could only be assigned in the Eastern Shore.

The partial overlay ended in 1990, but it soon became apparent that this would not free up enough numbers to meet demand. The supply of available numbers was further limited by the fact that most of the Maryland side of the Washington area shares an LATA with northern Virginia and the District itself.

By the fall of 1990, it was apparent that there was no way to stave off the immediate need for a new area code in Maryland. In November 1990, a plan to add a second area code to the state was announced. It was decided that the Baltimore metropolitan area and the Eastern Shore would get the new area code, area code 410, while western and southern Maryland—including the Washington suburbs—would remain with the 301 area code.^[2] Because the bulk of Maryland's population resides in the Washington suburbs, this area retained the existing area code. Additionally, Bell Atlantic (now Verizon), the largest carrier in the region, wanted to spare the large number of federal agencies on the Maryland side of the Washington metro the expense and burden of having to change their numbers.

Area code 410 officially entered service on October 6, 1991; it was initially implemented in permissive-dialing mode, not as a flash-cut, with the 10-digit dialing scheme coming in for local calls across the new 301/410 boundary. The split largely followed metro lines. However, parts of Anne Arundel, Carroll, Frederick, and Howard counties were split between area codes 301 and 410.^[3][1] Effective November 1, 1991, ten-digit dialing was required when calling a different area code in Maryland.^[4]

Although the split was intended to be a long-term solution, within five years 410 was already close to exhaustion due to the area's rapid growth and the proliferation of cell phones and pagers. To solve this problem, area code 443 was overlaid onto the 410 territory on July 1, 1997. Overlays were a new concept at the time, and had met with some resistance due to the prospect of different area codes in the same area as well as the requirement for ten-digit dialing. However, even with 410 running out of numbers, the Eastern Shore was not nearly large enough for its own area code. In any event, a split would have forced its residents to change their numbers for the second time in a decade.

By 2011, the 410/443 area was once again running out of numbers due to the continued proliferation of cell phones. To prevent residents from having to change their phone numbers to a new area code, a third overlay, area code 667, was implemented on March 24, 2012.^[5] This had the effect of assigning 24 million numbers to just over four million people.

The counties served by these area codes include: