We studied an underutilized source of data on legislative effectiveness and exploited its panel structure to uncover several interesting patterns. We found that effectiveness rises sharply with tenure, at least for the first few terms, even when we control for legislators' institutional positions, party affiliation, and other factors. Effectiveness never declines with tenure, even out to nine terms. The increase in effectiveness is not simply due to electoral attrition and selective retirement, but to learning-by-doing. We also found evidence that a significant amount of “positive sorting” occurs in the legislature, with highly talented legislators moving more quickly into positions of responsibility and power. Finally, effectiveness has a positive impact on incumbents' electoral success and on the probability of legislators moving to higher office. These findings have important implications for arguments about term limits, the incumbency advantage, and seniority rule.