Same-sex sexual interactions (SSSI) have been observed in many animal groups and have intrigued evolutionists. In this paper, reports on SSSI in Lepidoptera are reviewed and evolutionary hypotheses that could explain these behaviors are discussed. SSSI have been documented in males of 25 species and in females from role-reversed populations of one species. Four types of SSSI have been reported: pupal guarding, courtship, copulation attempt, and copulation. Although the hypotheses cannot be tested with the limited data, evidence suggests that in some Lepidoptera SSSI could result from selection for imposing costs on other males, or could be a by-product of sexual selection favoring individuals that exhibit high sexual willingness. In agreement with both hypotheses, in the 17 species whose mating systems are known, there is intense competition for mates in the sex exhibiting SSSI. We propose lines of research on SSSI in Lepidoptera.