An overview is given of Formula (an abbreviation for Forth Music Language), a language for controlling synthesizers that can model the expressiveness of a human performance. Formula supports algorithmic composition, interactive performance, and programmed interpretation of traditional scores. It uses concurrent processes that share a single address space and are scheduled by the runtime system. Note-playing processes compute sequences of pitches and play these pitches as notes or chords. Auxiliary processes are attached to note-playing processes or groups to supply note parameters such as volume, duration, and articulation. Input-handling processes execute when input arrives from a particular device. Two representative Formula programs are described.