The steel-string acoustic guitar is a modern form of guitar that descends from the gut-strung Romantic guitar, but is strung with steel strings for a brighter, louder sound. Like the modern classical guitar, it is often referred to simply as an acoustic guitar, or sometimes as a folk guitar.

The most common type is often called a flat top guitar, to distinguish it from the more specialized archtop guitar and other variations.

The standard tuning for an acoustic guitar is E-A-D-G-B-E (low to high), although many players, particularly fingerpickers, use alternate tunings (scordatura), such as open G (D-G-D-G-B-D), open D (D-A-D-F♯-A-D), drop D (D-A-D-G-B-E), or D-A-D-G-A-D (particularly in Irish traditional music).

Steel-string guitars vary in construction and materials. Different woods and approach to bracing affect the instrument's timbre or tone. While there is little scientific evidence, many players and luthiers believe a well-made guitar's tone improves over time. They theorize that a decrease in the content of hemicellulose, crystallization of cellulose, and changes to lignin over time all result in its wood gaining better resonating properties.

Steel-string acoustic guitars are commonly constructed in several body types, varying in size, depth, and proportion. In general, the guitar's soundbox can be thought of as composed of two mating chambers: the upper bouts (a bout being the rounded corner of an instrument body) on the neck end of the body, and lower bouts (on the bridge end). These meet at the waist, or the narrowest part of the body face near the soundhole. The proportion and overall size of these two parts helps determine the overall tonal balance and "native sound" of a particular body style – the larger the body, the louder the volume.

Any of these body type can incorporate a cutaway, where a section of the upper bout below the neck is scalloped out. This allows for easier access to the frets located atop the soundbox, at the expense of reduced soundbox volume and altered bracing, which can affect the resonant qualities and resulting tone of the instrument.

All of these relatively traditional looking and constructed instruments are commonly referred to as flattop guitars. All are commonly used in popular music genres, including rock, blues, country, and folk.

Other styles of guitar which enjoy moderate popularity, generally in more specific genres, include: