How to define and identify sexual orientations for the purpose of constructing representative samples of homosexuals, bisexuals, and heterosexuals is unclear and confusing to researchers. Different definitions and measures have been proposed and used to develop samples since the 1860s when sexual orientations first gained widespread research interest. Unfortunately, the definitions and measurement tools used since then result in the selection of divergent and incomparable samples. If advances in the understanding of sexual orientations are to be made, it is critical that definitions and measures of sexual orientation be standardized. This paper reviews and critiques definitions and measures of sexual orientation that have been proposed and used by researchers over the past century. This review is intended to further our understanding of this subject and to encourage researchers to be critical of how they classify subjects based upon sexual orientation.