Operational Type Theory (OpTT) is a type theory allowing possibly diverging programs while retaining decidability of type checking and a consistent logic. This is done by distinguishing proofs and (program) terms, as well as formulas and types. The theory features propositional equality on type-free terms, which facilitates reasoning about dependently typed programs. OpTT has been implemented in the GURU verified programming language, which includes a type-and proof-checker, and a compiler to

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... ficient C code. In addition to the core OpTT, GURU implements a number of extensions, including ones for verification of programs using mutable state and input/output. This paper gives an introduction to verified programming in GURU.