The aim to deliver multiple forms of value to a variety of stakeholders distinguishes sustainable business models (SBMs) from other types of business models. However, despite being central to defining SBMs, value itself is seldom defined. We argue that explicit definitions of value are necessary in each SBM research context, to advance theorizing and methodological rigour. Our findings from a four-stage study offer guidance for developing good definitions of value in SBMs. Firstly, our systematic literature review reveals that value is used in many different contexts with different implicit meanings, in a black-box approach that defines SBMs without defining value. Secondly, drawing on philosophical methodology literature, we identify criteria for valid and operational definitions. Thirdly, we apply these criteria to identify prevalent issues within existing attempts at defining value in SBM – including circularity, ambiguity, and vagueness. Based on our conclusion that the value concept may be a ‘real vague’ which can never be precisely or completely defined, we emphasize the need for SBM researchers to explicitly define value in each research context. Finally, we provide an analytical framework to guide the development of good definitions of value in SBMs, and suggestions for further research into SBM theorizing methodology.