For the cow, fertilisation rates are generally reported to be close to 90 per cent while average calving rates fall within the range of 50 to 60 per cent. Thus, embryonic and fetal deaths account for about 75 per cent of all reproductive wastage, following a single breeding. Early embryonic death (within 20 days of fertilisation) would seem to account for about 75 to 80 per cent of all embryonic and fetal deaths and results in a substantial loss in production. The literature relating to some possible genetic and environmental causes of embryo death is briefly reviewed. The relationship between progesterone level and the initiation and maintenance of pregnancy is discussed and some data are presented.