Merit pay programs for educators -- sometimes referred to a "pay for performance" -- attempt to tie a teacher's compensation to his/her performance in the classroom. While the idea of merit pay for classroom teachers has been around for several decades, only now is it starting to be implemented in a growing number of districts around the country. One example of the increased interest for merit pay systems can be seen in the the recent increased funding level for the federal Teacher Incentive Fund (TIF). The TIF program, which is run through the United States Department of Education (USDOE), provides funding to school districts to help them implement merit pay systems. The USDOE has increased funding for the TIF program this year by more than four-fold -- from $97.3 million to $437 million. But with all of this increased interest and funding for merit pay programs -- what if anything is known about the costs versus the benefits of these systems? This issue of "The Progress of Education Reform" reviews what is known and unknown about teacher merit pay systems. (Contains 7 endnotes.)