This book offers a distinctive perspective on keyword expertise by positioning teacher expertise within broader social and political debates surrounding the constitution of knowledge, truth and experts. Bringing debates surrounding teacher expertise into conversation with wider debates surrounding expertise, Jessica Gerrard and Jessica Holloway reflect on recent events, including COVID-19 and the climate crisis, to show that expertise is not a neutral, objective or fixed idea that is easily definable. Rather than seek to understand expertise simply in relation to pedagogical or curriculum practices in the classroom, they demonstrate that any account of expertise must first reckon with its social and political positioning and how it creates boundaries between who is an expert and who is not. Drawing on international research, this book places experience within a global context, attending to the ways in which teacher expertise itself is subject to struggles and contestations in power in different ways across different national and sub-national contexts. Through illustrative examples taken from this research, this book shows how expertise is as much about who is understood to be an expert, and who is understood not to be. Expertise sheds timely light on the contradictory and ambivalent practices of expertise.