There are three aspects of learning that we should consider when we are trying to optimize performance. First is the efficiency of training. Because of the high costs of training, we certainly want to be sure that training is accomplished as quickly as possible. However, optimizing training speed should not be the only, or even the most important, goal. If individuals have successfully learned how to perform a task during training but then forget how to perform it some time later, the training has clearly been ineffective. Passing a test at the end of training does not guarantee later success in the field. Long-term retention of the trained knowledge and skills is essential. Learning needs to be durable as well as efficient. But even durable training will not be sufficient if the learned knowledge and skills cannot be applied to situations different from those encountered during training. Training can rarely capture the full set of circumstances under which tasks are subsequently encountered. Another important goal for learning then is transfer or flexibility. Thus, optimal learning should be efficient, durable, and flexible. This chapter discusses research on the training, retention, and transfer of knowledge and skills, and also reviews research that we have been conducting on training for efficient, durable, and flexible performance in the military.