HO scale (H0 scale in continental Europe) is the most popular scale of model railway in most of the world outside the United Kingdom, where the slightly larger in scale OO gauge is most common. The name is derived from the German Halb-null ("half-zero"), because its 1:87 scale is approximately half that of O scale.

Modern HO trains run on realistic-looking two-rail track, which is powered by direct current (varying the voltage applied to the rails to change the speed, and polarity to change direction), or by Digital Command Control (sending commands to a decoder in each locomotive). Some trains, most notably by Märklin of Germany, run on alternating current, supplied by a "third rail" consisting of small bumps on each tie down the centre of the track.

HO scale trains first appeared in the United Kingdom in the 1930s, originally as an alternative to OO scale. It proved unsuitable for scale modelling UK trains. However, it became very popular in the United States, where it took off in the late 1950s after interest in model railroads as toys began to decline and more emphasis began to be placed on realism in response to hobbyist demand. While HO scale is by nature more delicate than O scale, its smaller size allows modelers to fit more details and more scale miles into a comparable area.

In the 1960s, as HO scale began to overtake O scale in popularity, even the stalwarts of other sizes, including Gilbert (makers of American Flyer) and Lionel Corporation began manufacturing HO trains. HO locomotives, rolling stock (cars or carriages), buildings and scenery are available today from a large number of manufacturers in a variety of price brackets.