The fluency of basic arithmetical operations is a precondition for mathematical problem solving. However, the training of skills plays a minor role in contemporary mathematics instruction. The authors proposed individualization of practice as a means to improve its efficiency, so that the time spent with the training of skills is minimized. As a tool to relieve teachers from the time-consuming tasks of individual diagnosis, selection of problems, and immediate feedback, they developed adaptive training software. The authors evaluated the application of the software in 2 naturalistic studies with 9 third-grade classes. Results show that even a moderate amount of individualized practice was associated with large improvements of arithmetic skills and problem solving, even after a follow-up period of 3 months. (Contains 3 tables, 3 figures and 1 note.)