This paper introduces matching logic, a novel framework for defining axiomatic semantics for programming languages, inspired from operational semantics. Matching logic specifications are particular first-order formulae with constrained algebraic structure, called patterns. Program configurations satisfy patterns iff they match their algebraic structure and satisfy their constraints. Using a simple imperative language (IMP), it is shown that a restricted use of the matching logic proof system is equivalent to IMP's Hoare logic proof system, in that any proof derived using either can be turned into a proof using the other. Extensions to IMP including a heap with dynamic memory allocation and pointer arithmetic are given, requiring no extension of the underlying first-order logic; moreover, heap patterns such as lists, trees, queues, graphs, etc., are given algebraically using fist-order constraints over patterns.