This article investigates the work of individuals who help out informally with a family member's job, often without pay. Examples include the relative who works in the back room of the family business, the executive spouse who hosts corporate functions, the political wife who campaigns with her husband, or the child who does chores on the family farm. The term "unpaid market labor" (UML) is used here to describe the ways that family members collaborate directly in paid activities that are legally and socially attributed to others. The practical legal problems that can arise in relation to UML are illustrated in the context of Canadian and U.S. tax cases regarding the distinction between business and personal activities. The article surveys empirical evidence about the nature and extent of UML undertaken in industrialized countries, and recent studies which suggest that family collaboration in breadwinning may be growing in response to the pressures of economic globalization, technological change, and labor market restructuring. The author proposes a framework for incorporating an analysis of UML into feminist critiques of the market/family dichotomy in law, and responds to possible concerns that doing so may commodify family relations or implicitly devalue unpaid care work.