A study conceptualized bases for learning in workplaces, examining reciprocity between how individuals are afforded access to workplace activities and guidance and how workers elect to engage in what they are offered. These reciprocal bases for thinking, acting, and learning are called "co-participation at work." The contributions that workplaces make to workers' learning primarily comprise the structuring of learners' access to workplace activities and guidance. These affordances or invitational qualities include how workplace activities and guidance are distributed and accessed, and by whom. Reciprocally, beyond what the workplace offers, what workers learn and how they draw upon and transform their ways of knowing about work is shaped by how they elect to engage in work activities and construe the invitational qualities of what is afforded. Recent accounts of reciprocity in relations between mind and social practice have emphasized the interdependencies or relatedness between individuals' acting and social practice, including the workplaces in which they act. A key premise is that engagement in work and what is learned through work are shaped at the intersection between the evolving social practice of the workplace and individuals' ongoing development founded in their ontogeneses. Therefore, an understanding of participation in and learning at work illuminates relations between social and cognitive contributions to adult development throughout working lives. (Contains 48 references.) (Author/KC)