Pleiotropy is defined as one locus exerting an influence on more than one phenotype. Initial studies into pleiotropy sought to distinguish “genuine” pleiotropy (one gene with multiple products) from “spurious” pleiotropy (one unique gene product with different effects on the organism). Pleiotropy can be defined on many scales and levels whether the unit defined is at a cellular, tissue, individual, or population scale. Much of the work currently being carried out on the measurement and analysis of pleiotropy is based on large genomic datasets, and initial results suggest that when a gene is pleiotropic it is highly modular, affecting a small set of traits. Current challenges are mainly linked to the problem of detectability and statistical power.