Embryonic mortality is an important cause of reproductive losses in the pig. The embryonic mortality rate, 30 per cent on average, will be reduced under optimal management conditions if (1) all sows are inseminated between 10 and 25 hours before ovulation; (2) semen of boars of proven fertility is used; and (3) variation in embryonic development is small. The uterine capacity of sows should also improved to enable the lower mortality rate to be expressed in larger litters.