Background/Context: Teachers' mathematical knowledge has been the subject of recent federal policy, public programs, and scholarly attention. Numerous reports identify a need for improving this knowledge, and total federal spending on content-focused math and science professional development during the period 2002-2007 is estimated to be above $1.2 billion. Purpose/Objective/Research Question/Focus of Study: In this study, we investigated the patterns in and the effect of professional development in the area of middle school mathematics. We focus in particular on professional development thought to influence teachers' mathematical knowledge, and use a measure of such knowledge to gauge potential growth in a sample of teachers included in a national survey. Research Design: We surveyed a nationally representative sample of roughly 1000 teachers in both 2005 and 2006 and obtained responses from 461 middle school teachers at both time points. The survey measured their mathematical knowledge for teaching at both time points and also inquired about their professional learning opportunities during the intervening year. Data Collection and Analysis: We used survey data both to describe the nature of teachers' learning opportunities during 2005-2006 and to associate participation in these learning opportunities with teachers' 2005 knowledge scores as well as other characteristics. We also linked these learning opportunities to observed growth in teacher knowledge. Findings/Results: Results indicate that extensive effort and expenditures have not dramatically transformed teacher professional development practices as compared to past descriptions, in that learning opportunities reported by teachers are still typically short and fragmented. However, there are indications that teachers' mathematical knowledge might have improved somewhat during this time period. Conclusions/Recommendations: Results from this study suggest a very mixed picture of the middle school mathematics professional development system. Effects on teacher knowledge are modest, and critics might claim that money would be better spent in hiring or induction programs.