AbstractWe introduce the idea of optimisation validation, which is to formally establish that an instance of an optimising transformation indeed improves with respect to some resource measure. This is related to, but in contrast with, translation validation, which aims to establish that a particular instance of a transformation undertaken by an optimising compiler is semantics preserving. Our main setting is a program logic for a subset of Java bytecode, which is sound and complete for a resource-annotated operational semantics. The latter employs resource algebras for measuring dynamic costs such as time, space and more elaborate examples. We describe examples of optimisation validation that we have formally verified in Isabelle/HOL using the logic. We also introduce a type and effect system for measuring static costs such as code size, which is proved consistent with the operational semantics.