This article attempts to apply mastery learning theory to research on full- and half-day kindergarten. It is proposed that the application of mastery learning theory would greatly aid in the understanding of relationships between important program variations and predicted outcomes, and demonstrably enhance research efforts to identify differences between full- and half-day programs. It is further argued that, in a more general fashion, the use of mastery learning theory would help quantify essential dimensions of kindergarten classroom practice, and thereby might contribute to a resolution of what many regard as the unclear role of kindergarten education. The discussion includes a brief review of key concepts of mastery learning, and focuses on the qualities which make the theory especially suitable for application to kindergarten issues. A reconceptualization of strategies for full- and half-day kindergarten research is presented. Potential criticisms or reservations are discussed. (Author/RH)