There is currently no clear operational conceptualization or metric of a “sustainable diet”. We aimed to identify the range of conceptualizations of and approaches to measurement of sustainable diets through a comprehensive, systematic review of the scientific and grey literatures. Two independent reviewers screened the titles and abstracts of 719 articles that were identified through the systematic searching of 15 databases using a uniform set of search terms focused on use of the term “sustainable diet”. In total, 95 empirical and conceptual articles were included in the final review. Few empirical studies have attempted to directly measure the sustainability of diets. We identified only 24 such studies, nearly all (n=23) centered on high-income countries. All but five of the identified studies assessed sustainability through measurement of greenhouse gas emissions (GHGE) associated with the production, processing, and transport of a limited variety of foods. Six studies considered the nutritional quality of diets in addition to GHGE and these primarily emphasized differences between meat-based and vegetarian diets. Five studies examined the economic costs of diets alongside environmental considerations. Conceptualizations of the sustainability of diets spanned a range of environmental, social, economic, and health-related domains. Empirical data measuring the sustainability of diets are limited, particularly in low- and middle-income countries, and emphasize only one of several principal domains of sustainability.