In this second report on quantitative methods, I consider the foundational triumvirate that underpins the sub-discipline: data, methods, and theory. I argue that, although ‘big data’ and, more recently, ‘big code’ may have captured the limelight, theory is invaluable and should not be disregarded. Showcasing the integral role of theory in quantitative human geography, I identify three kinds of useful theory (old school, enduring and conceptual) and, along the way, highlight beneficial ways in which they contribute to our thinking about data and methods. Throughout, I emphasize that theory is a crucial component of contemporary and future quantitative methods development.