This chapter examines the progress in developing radiotracers for selectively probing two enzymes, hexokinase and monoamine oxidase (MAO). It describes some of the general problems and pitfalls in developing positron emitter labeled radiotracers whose uptake in vivo reflects catalytically active enzyme. The suicide enzyme inactivators are enzyme substrates which contain a latent reactive functional group which is unmasked within the enzyme-substrate complex during the catalytic step. The chapter explains the development of specific positron emitter labeled radiotracers to map MAO. MAO exists in two forms, MAO A and MAO B, which are identified by their substrate selectivity and their sensitivity to different inhibitors. The preceding sections serve to illustrate some of the complexities involved in developing radiotracers for probing a specific biochemical transformation using positron emission tomography (PET). Some of the approaches have been applied to the development of positron emitter labeled tracers for probing monoamine oxidase with PET.