The availability of objective quantitative measures of a player’s chess strength makes chess an optimum field to study individual differences. While several indicators quantify accurately chess skill, the Elo rating system is by far the most popular and worldwide-accepted chess skill indicator. Every chess player participating regularly in rated tournaments holds an Elo rating, a dynamic indicator that depends on the outcomes in the games played within a given time period considering the Elo rating of the opponents. The chapter provides an in-depth description of the Elo chess rating system, while highlighting valid alternatives such as the Universal rating system (URS). The quantitative rating of chess skill has been used in a variety of studies addressing relevant topics in several fields of psychological research, which are summarized within the Appendix 1 of the book.