Since the middle of the twentieth century, hundreds of programming languages have been designed and implemented – and new ones are continually emerging. The syntax of a programming language can usually be described quite precisely and efficiently using formal grammars. However, the formal description of its semantics is much more challenging. Language designers, implementers and programmers commonly regard formal semantic descriptions as impractical. Research in semantics has allowed us to reason about software and has provided valuable insight into the design of programming languages, but few semantic descriptions of full languages have been published, and hardly any of these are currently available online.