K is a rewrite-based executable semantic framework in which programming languages, type systems and formal analysis tools can be defined using configurations and rules. Configurations organize the state in units called cells, which are labeled and can be nested. K rewrite rules make it explicit which parts of the term are read-only, write-only, read-write, or unused. This makes K suitable for defining truly concurrent languages even in the presence of sharing. Computations are represented as syntactic extensions of the original language abstract syntax, using a nested list structure which sequentializes computational tasks, such as program fragments. Computations are like any other terms in a rewriting environment: they can be matched, moved from one place to another, modified, or deleted. This makes K suitable for defining control-intensive features such as abrupt termination, exceptions, or call/cc.