Dylan is a multi-paradigm programming language that includes support for functional and object-oriented programming (OOP), and is dynamic and reflective while providing a programming model designed to support generating efficient machine code, including fine-grained control over dynamic and static behaviors. It was created in the early 1990s by a group led by Apple Computer.

Dylan derives from Scheme and Common Lisp and adds an integrated object system derived from the Common Lisp Object System (CLOS). In Dylan, all values (including numbers, characters, functions, and classes) are first-class objects. Dylan supports multiple inheritance, polymorphism, multiple dispatch, keyword arguments, object introspection, pattern-based syntax extension macros, and many other advanced features. Programs can express fine-grained control over dynamism, admitting programs that occupy a continuum between dynamic and static programming and supporting evolutionary development (allowing for rapid prototyping followed by incremental refinement and optimization).

Dylan's main design goal is to be a dynamic language well-suited for developing commercial software. Dylan attempts to address potential performance issues by introducing "natural" limits to the full flexibility of Lisp systems, allowing the compiler to clearly understand compilable units, such as libraries.

Dylan derives much of its semantics from Scheme and other Lisps; some Dylan implementations were initially built within extant Lisp systems. However, Dylan has an ALGOL-like syntax instead of a Lisp-like prefix syntax.

Dylan was created in the early 1990s by a group led by Apple Computer. At one time in its development, it was intended for use with the Apple Newton computer, but the Dylan implementation did not reach sufficient maturity in time, and Newton instead used a mix of C and the NewtonScript developed by Walter Smith. Apple ended their Dylan development effort in 1995, though they made a "technology release" version available (Apple Dylan TR1) that included an advanced integrated development environment (IDE).

Two other groups contributed to the design of the language and developed implementations: Harlequin released a commercial IDE for Microsoft Windows and Carnegie Mellon University released an open source compiler for Unix systems called Gwydion Dylan. Both of these implementations are now open source. The Harlequin implementation is now named Open Dylan and is maintained by a group of volunteers, the Dylan Hackers.

The Dylan language was code-named Ralph. James Joaquin chose the name Dylan for "DYnamic LANguage."

Many of Dylan's syntax features come from its Lisp heritage. Originally, Dylan used a Lisp-like prefix syntax, which was based on s-expressions. By the time the language design was completed, the syntax was changed to an ALGOL-like syntax, with the expectation that it would be more familiar to a wider audience of programmers. The syntax was designed by Michael Kahl. It is described in great detail in the Dylan Reference Manual.