The book introduces a framework for examining the validity of tests that aim to assess second-language (L2) proficiency development over time and/or in relation to L2 instruction. It also reports the findings of a longitudinal study that aimed to examine the sensitivity to change of a test of L2 proficiency development. Specifically, the study examined changes over time in Progress scores and the linguistic characteristics of essays written in response to Progress by learners who took the test before, during and after a period of L2 instruction in different countries. The book furthers our understanding of the nature of L2 proficiency as it develops over time and in relation to L2 instruction and provides a framework that can be used in future endeavours to design and validate tests of L2 proficiency development. The book is intended for graduate students, test developers, and researchers doing research in applied linguistics and L2 assessment.