Pleiotropy is the effect resulting from an interconnected genetic system in which a single gene influences many different biological systems in positive or negative ways. Pleiotropy occurs because multiple phenotypic effects usually arise from each expressed gene. The adverse effects of pleiotropy on the effectiveness of natural selection is reviewed in this paper. It is concluded that pleiotropy creates a major problem for evolutionary theory because the accumulation of mutations, even beneficial mutations, often has unintended negative effects.