Market actors see circular economies as critical in responding to the climate emergency. Circular economies are systems that collectively mitigate environmental harm. Recent research suggests that we know little about how circular economies are built in everyday market practice. Market studies have focused on the making and shaping of market transactions, and the connections needed to make things valuable. However, they do not provide sufficient insights into how market actors build market circularities. We do not understand how actors evaluate and make valuable market circularities, so that they consider them worth engaging with. I introduce the concept of ‘circular markets’ to foreground the work done by market actors to keep materials in circulation, so that they continue to generate socio-economic value, instead of being categorised as ‘waste’. I argue that ‘circular markets’ provide valuable insights into the valuation practices performed to make and connect markets, building a circular economy: one connection at a time. Drawing on an ethnography of electric vehicle battery markets and their associated actors, conducted between December 2017 and January 2022, the thesis uncovers aspects in the production and consumption of valuations that make and connect circular markets. Articulating these valuations, the thesis comprises three research papers. Paper 1 investigates market actors’ attempts to agence circular market connections. Paper 2 explores how market actors compete to shape transactions as part of a programme of circular market action. Paper 3 reflects on my performance of methods as I participated in circular market action. The thesis contributes to knowledge of how to build circular economies via an in-depth analysis of the numerous practices needed to constitute circular markets. I propose a circular markets agenda for market studies to equip market actors with both the knowledge and analytical know-how to mobilize stronger responses to the climate emergency through market action.