The role of prior knowledge in learning complex procedures was investigated in a transfer task in which subjects learned two related procedures in sequence. In Experiment 1, we manipulated the conceptual and structural similarity between the two procedures; in Experiment 2, we manipulated whether the order of the steps within subprocedures was the same or different during training and transfer, or whether the order of the subprocedures was the same or different. The results lead us to hypothesize that transfer in complex procedures is mediated primarily by a memory for specific steps rather than by conceptual understanding or problem solving. In particular, we were able to model the results precisely on the assumption that subjects use superficial similarity to retrieve the sequences of steps needed to perform segments of the procedure.People successfully perform a variety of complex procedures every day, ranging from relatively mundane activities such as baking a cake or balancing a checkbook, to more technology-dependent procedures such as programming a VCR or using a fax machine. And although there are a few notorious counterexamples, even novel procedures are generally learned quickly with little apparent effort. For example, cooks typically can use a new recipe with ease, and a new model of banking machine usually poses no problem to a user. Our view is that the success that is typically enjoyed in such situations is a function of prior knowledge: Novel procedures are acquired easily when people are already familiar with similar procedures. Inthe present research, we were concerned with the nature of this prior knowledge and how it is used in acquiring new procedures.Previous research has suggested that two kinds ofknow1-edge might be involved in learning new procedures: conceptual knowledge and structural knowledge. Conceptual knowledge refers to information about why a procedure works, what a device does, or how it operates. For example, in learning to operate a VCR, relevant conceptual knowledge might include the fact that information is recorded on magnetic tape in the cassette, that the tape is transported from the one side of the cassette to the other during playing and recording, that VCR operations can be controlled by a programmable timer, and so on. On the other hand, structural knowledge refers to knowledge about the structure of the procedures: the steps that have to be performed, the order of the steps, their organization into subtasks, and the rules that determine legal sequences of steps. For example, structural knowledge regarding VCRs This research was supported by a Natural Sciences and Engineering Research Council of Canada research grant to the first author. Siobhan Nearey is now with the Department of Psychology, Carnegie Mellon University, Pittsburgh, PA 15213. Correspondence should be addressed to P. Dixon, Department of Psychology, University of Alberta, Edmonton, AB, Canada T6G 2E9 (e-mail: pdixon@psych.ualberta.ca). might include the fact that there are procedures for setting the t...