Diffraction: QCD Effects in Colour Singlet Exchange

Measurements of diffractive phenomena observed at HERA and the Tevatron are reviewed. A short introduction to the theoretical background is presented where colour singlet exchange reactions are discussed and the diffractive contribution and its interpretation via pomeron exchange outlined. The review focuses on the current experimental directions at HERA and discusses exclusive production of vector mesons, the dissociation of real photons and the deep inelastic structure of diffraction. Complementary information obtained from hadronic final states and jet structures is also discussed. The experimental signatures for diffractive jet and $W^\pm$ production observed at the Tevatron are described and the rates compared with those from the HERA experiments.