Two large, diverse samples of tournament-rated chess players were asked to estimate the frequency and duration of their engagement in a variety of chess-related activities. Variables representing accumulated time spent on serious study alone, tournament play, and formal instruction were all significant bivariate correlates of chess skill as measured by tournament performance ratings. Multivariate regression analyses revealed that among the activities measured, serious study alone was the strongest predictor of chess skill in both samples, and that a combination of various chess-related activities accounted for about 40% of the variance in chess skill ratings. However, the relevance of tournament play and formal instruction to skill varied as a function of skill measurement time (peak vs. current) and age group (above vs. below 40 years). Chess players at the highest skill level (i.e. grandmasters) expended about 5000 hours on serious study alone during their first decade of serious chess play—nearly five times the average amount reported by intermediate-level players. These results provide further evidence to support the argument that deliberate practice plays a critical role in the acquisition of chess expertise, and may be useful in addressing pedagogical issues concerning the optimal allocation of time to different chess learning activities. Copyright © 2005 John Wiley & Sons, Ltd.