Combinatory logic was invented by Moses Schönfinkel in the early 1920s, and was mostly developed by Haskell Curry. The idea was to reduce the notation of logic to the simplest terms possible. As such, combinatory logic consists only of combinators, combination operations, and no free variables.

A combinator is simply a function with no free variables. A free variable is any variable referred to in a function that is not a parameter of that function. The operation of combination is then simply the application of a combinator to its parameters. Combination is specified by simple juxtaposition of two terms, and is left-associative. Parentheses may also be present to override associativity. For example

All combinators in combinatory logic can be derived from two basic combinators, and . They are defined as

Reference is sometimes made to a third basic combinator, , which can be defined in terms of and .

Combinatory logic where is considered to be derived from and is sometimes known as pure combinatory logic.

Combinatory logic and lambda calculus are equivalent. However, lambda calculus is more concise than combinatory logic; an expression of size in lambda calculus is equivalent to an expression of size in combinatory logic.

For example, in combinatory logic is equivalent to , and is equivalent to .