Consistent production of large quantities of fertile semen is a primary concern of boar management programmes. Common visual tests such as motility are poor indicators of the fertilizing capacity of an ejaculate. Research studies have demonstrated that, in general, if motility is 60% or greater, there are no relationships among the percentage of motile spermatozoa, in vitro sperm penetration rates, farrowing rates and litter size. Biological and environmental factors that influence the production of total numbers of spermatozoa are either inhibitory or stimulatory. Chronic undernutrition and heat stress inhibit spermatogenesis, while photoperiod and genetic factors are stimulatory. A consistent stimulatory effect of photoperiod does not appear to occur under all environmental conditions. In contrast, selection for increased testis size consistently results in greater sperm production. In addition, libido is decreased by heat stress and undernutrition. However, failure to allow boars to interact socially with other pigs during pubertal development appears to be the most damaging factor. Effective boar management programmes must minimize the occurrence of factors that adversely affect spermatogenesis and simultaneously accentuate those with a stimulatory role.