More than a decade has passed since a conceptual framework was introduced to guide public health services and systems research (PHSSR) and elucidate the relationships associated with system performance. Since then, research has primarily focused on performance, standards, and key processes, with less emphasis on identification of measures or methods. Capacity lies at one end of the conceptual framework, although little emphasis has been placed on measuring and defining “capacity” of the public health system. This is striking, given organizational capacity is a critical determinant of performance and is necessary for understanding systematic effectiveness, sustainability, or generalizability. As a nascent field, PHSSR needs to develop a definition of organizational capacity and elucidate its relationship within a research framework. Evidence must be developed on the temporal and causal relationships between capacity, process/performance, and outcomes. The purpose of this article was to review research frameworks and capacity measures in various disciplines to expand the existing PHSSR conceptual framework.