The widespread global replacement of bulls with artificial insemination (AI) means that accurate detection and timing of oestrus becomes a critical activity. Traditional observation of behavioural signs have been re-evaluated, but these methods rely on regular visual observation of cows and an environment conducive to demonstration of oestrus behaviour. Dr Maggie Fisher BVetMed CBiol MSB MRQA DipEVPC MRCVS and Dr Peter Holdsworth AO FRSB FAICD, of Ridgeway Research Ltd, review some of the detection systems which have been commercialised and are widely adopted, whilst others remain in the research laboratory.