An apartment building is a structure where multiple families or people live in separate homes stacked on top of each other or arranged side by side. Each home in the building is called an apartment or unit, and has its own kitchen, bathroom, and living spaces, but residents share the building's entrance, hallways, and sometimes amenities like laundry rooms or playgrounds.

Apartment buildings come in many sizes. Some have just a few units in a three-story building, while others tower dozens of stories high with hundreds of apartments. In cities where land is expensive and scarce, apartment buildings make efficient use of space by building upward rather than outward.

People who live in apartment buildings are usually tenants who pay monthly rent to a landlord or property management company. Unlike owning a house, apartment dwellers don't own their individual units (though there's a similar arrangement called a condominium, where people do own their individual apartments). The building's owner or management handles exterior maintenance and repairs, while tenants take care of their own apartments.

In some countries, apartment buildings are called flats or blocks of flats. Some of the tallest apartment buildings in the world can be found in cities like New York, Hong Kong, and Dubai.