The average human ejaculate contains a quarter of a billion sperms. Are so many really needed for normal fertility? Hypotheses include massive redundancy correlated with crossing-over at meiosis and the potential challenges of sperm competition. The notion of human “sperm wars” will be reviewed in light of evidence derived from comparative studies across mammals of sperm production in relation to mating systems. Since 1974, there have been accumulating claims that human sperm counts are declining markedly in numerous industrialized populations. Differences in occurrence and timing between regions suggest environmental influences. Sperm counts have not yet declined to levels where fertility is severely threatened, but how serious is the problem and what might the future hold?