We present the first profiler for a compiled, non-strict, higher-order, purely functional language capable of measuring time as well as space usage. Our profiler is implemented in a production-quality optimising compiler for Haskell, has low overheads, and can successfully profile large applications. A unique feature of our approach is that we give a formal specification of the attribution of execution costs to cost centres. This specification enables us to discuss our design decisions in a precise framework. Since it is not obvious how to map this specification onto a particular implementation, we also present an implementation-oriented operational semantics, and prove it equivalent to the specification. 1 Motivation and overview Everyone knows the importance of profiling tools: the best way to improve a program's performance is to concentrate on the parts of the program which are eating the lion's share of the total space and time resources. One would expect profiling tools to be ...