This review critically examines the current state of the science on the concept of family caregiver–care receiver mutuality, summarizes accomplishments and gaps and identifies directions for future theory development and research. Mutuality between family caregivers and care receivers is of increasing interest to researchers. However, no analysis of the current state of the science of this important concept has been published. Our literature search revealed 34 research articles that met inclusion criteria. The studies were assessed in terms of conceptualization of mutuality, instrument development, populations studied, research designs and methods and findings. Significant scientific progress during the past 30 years includes the development of clear definitions and new instruments, expansion of research beyond the clinical populations in which mutuality was first studied, the use of a variety of research designs, and increasingly sophisticated methods of data analysis. Growing evidence suggests that mutuality is associated with caregiver emotional health outcomes and may decrease over time in the context of chronic illness. Directions for future research include development of new theoretical frameworks grounded in relational theory, development of theory on the dynamics of mutuality over time, exploration of mutuality in diverse cultures and populations, and intervention studies aimed at enhancing mutuality.