This chapter describes transfer as the use of knowledge or skill acquired in one situation in the performance of a new, novel task, a task sufficiently novel that it involves additional learning as well as the use of old knowledge. The importance of transfer in the study of learning cannot be overestimated. The system of education is based on the idea that knowledge and skills acquired in the classroom will be useful throughout life in the pursuit of meaningful work and in other intellectual activities: The value of classroom education, of industrial and military training, and of one's own programs of learning depend critically on the extent to which their designs are consistent with the truths about transfer. The chapter suggests three principles, which in combination with modern cognitive theory that helps researchers to look for transfer in more of the right places. The principles highlighted are old principles and are all related to careful analysis and understanding of the learner's task. In spite of their age, it is believed that they deserve more emphasis. It presents examples of three research projects that illustrate the utility of each of the principles in looking for transfer. The chapter concludes that the arguments about transfer have persisted because of the partial truth of both sides.