Language testing researchers have recently shown interest in diagnostic testing. However, diagnostic testing requires a better understanding of language abilities at a less general level than has been normal in language testing, and this has posed challenges to testers to define their diagnostic constructs both theoretically and operationally. In theory, second language acquisition (SLA) should be able to offer insights into the construct of reading in a second or foreign language (S/FL), and testing ought to be able to base diagnostic tests of S/FL reading on theoretical insights from SLA. It is, however, unclear whether this happens in practice. This paper examines the potential synergy between second language acquisition and second and foreign language testing, and reports on progress in three inter-related research projects into S/FL reading.