This collection of chapters is dedicated to the epistemology of measurement. As such, it marks a clear distinction from older debates in measurement theory. By the same token it advances a renewed interest in measurement as a key element of research practice. To the extent that it has been interested in epistemology at all, measurement theory has generally been concerned with questions of knowability. Norman R. Campbell (1920) famously asked what things have to be like in order to be measurable on a certain scale, and inversely, what we know about the world when we can measure some but not all things. Equally famously, Stanley S. Stevens (1946) started from the practice of assigning numbers to objects or events and left questions of metaphysics and epistemology to be answered implicitly through the advance of quantification. This is the point where measurement theory, properly speaking, entered in to determine the axiomatic conditions that need to be satisfied so that domains of objects become amenable to measurement (see Suppes et al. 2007).