Forward compatibility or upward compatibility is a design characteristic that allows a system to accept input intended for a later version of itself. The concept can be applied to entire systems, electrical interfaces, telecommunication signals, data communication protocols, file formats, and programming languages. A standard supports forward compatibility if a product that complies with earlier versions can "gracefully" process input designed for later versions of the standard, ignoring new parts which it does not understand.

The W3C's Technical Architecture Group (TAG) defines the concept more formally: "a change in the definition of a language is forward compatible if consumers of the original language can correctly process text written for the evolved version of the language".

The objective for forward-compatible technology is for old devices to recognise when data has been generated for new devices.

Forward compatibility for the older system usually means backward compatibility for the new system, i.e. the ability to process data from the old system; the new system usually has full compatibility with the older one, by being able to both process and generate data in the format of the older system.

Forward compatibility is not the same as extensibility. A forward compatible design can process at least some of the data from a future version of itself. An extensible design makes upgrading easy. An example of both design ideas can be found in web browsers. At any point in time, a current browser is forward-compatible if it gracefully accepts a newer version of HTML, whereas how easily the browser code can be upgraded to process the newer HTML determines how extensible it is.

The W3C Technical Architecture Group has documented a family of substitution rules that enable forward-compatible language design. Each rule specifies how a consumer should behave when it encounters constructs it does not recognize:

These rules underlie forward compatibility in widely deployed languages and protocols. HTTP/1.1, for example, specifies that unrecognized header fields should be ignored by the recipient and must be forwarded by transparent proxies — a direct application of the "preserve unknowns" rule.

The introduction of FM stereo transmission, or color television, allowed forward compatibility, since monophonic FM radio receivers and black-and-white TV sets still could receive a signal from a new transmitter. It also allowed backward compatibility since new receivers could receive monophonic or black-and-white signals generated by old transmitters.^{[citation needed]}