This paper reviews the current status and evolving issues in the field of zygosity testing. Monozygotic twins are seldom absolutely identical. Traditional methods of zygosity testing, mathematical ones and assessment of physical characteristics are reviewed. The proper delineation of zygosity based on placentation and chorionicity is outlined. Genetic and environmental discordance patterns are described, with a discussion of why "identical" twins are never identical preceding specific guidelines for clinical practice.