The aim of this study was to examine how the six teachers changed (or not) their professional knowledge of teaching (here referred to as PCK) after inquiring into their own teaching within the three learning studies. The data used in the article consisted of a video-recorded lesson from the four teachers before the project (PCK pretest) and after the project (PCK post-test), allowing an analysis of if and if then how the teachers changed their actual teaching practice. As such, this study responds to the call of Ball, Hill, and Bass (2005) to focus direct attention on the actual practice of teaching (and not on post-teaching reflection which is often used as data to capture PCK). The research question addressed is: In what way does learning study change teachers’ PCK?