The purpose of a learner is to make changes in a performance system to do similar tasks more effectively the next time. The meaning of "more effectively" can be only asserted in the context of the learner’s goals. Three types of learning goals can be distinguished in the context of planning systems: domain goals, planning efficiency goals, and plan efficiency or plan quality goals. Most work to date on learning for problem solving has focused on the first two types. We present a learning mechanism that focuses on the third type, namely learning control knowledge to improve plan quality, and give some preliminary results on the use of this method in a process planning domain.