Research studies indicate that experienced teachers are more effective, but also suggest multiple explanations why this might be the case – whether because teachers gain valuable skills on the job and through formal professional development opportunities, or because the least effective teachers tend to quit teaching earlier, while more effective teachers remain in the profession. Each of these possible reasons has distinct implications for policy: from increasing hiring standards, improving teacher training and raising the attractiveness of the teaching profession, to ensuring that novice teachers receive the necessary support to quickly learn the tools of the trade and taking measures to prevent good teachers from dropping out of the profession.