An apartment building, block of flats or tenement is a multi-unit dwelling made up of several (generally four or more) apartments (US) or flats (UK). Where the building is a high-rise construction, it is termed a tower block in the UK and elsewhere. The term apartment building is used regardless of height in the US.

A two-unit dwelling is known as a duplex (US) or maisonette (UK); a three-unit dwelling is known as a triplex.

United States

Apartment buildings are very popular multi-story buildings where three or more residences are contained within one structure. These apartment buildings can be found in almost every city in the United States with the exception of the most rural areas. Apartment buildings in suburban areas are generally occupied with tenants that either hold low or minimum wage jobs, or families just starting out and not yet able to afford a house mortgage. In more urban areas, an apartment close to the downtown area is preferred rather than a suburban house due to their proximity to their job, and/or public transportation. The distinction between apartments and condominiums is that while apartment buildings are owned by a single entity and rented out to many, condominiums are owned individually, while still paying a monthly or yearly fee for building upkeep. Condominums are often leased by their owner, giving the impression of being apartments.

In the United States, "tenement" is a label usually applied to the least expensive, most basic apartment blocks in older sections of large cities. The connotations associated with the word are poverty, crowding, and lack of amenities. Similar in nature to tenement housing and equally stigmatized are public housing projects, usually refered to as "the projects".

Scotland

During the 19th century tenements became the predominant type of new housing in Scotland's industrial cities. (In Northern England, 'back-to-back' terraces were more common). Scottish tenements are usually four to six stories in height, with three or four flats on each floor. They are sometimes still referred to as closes or closies (a reference to the passageway through which entry is gained). Stairs and landings are generally designated 'common areas', so residents must take it in turns to sweep clean the floors (this rarely happens in practice). Tenement flats are the most common form of accommodation for students who have moved out of University Halls (dorms).

Many multi-storey tower blocks were built in the UK after the Second World War. These are gradually being demolished and replaced with low-rise buildings or housing estates. In Scotland those that remain are usually called simply multis.