Abstract : This report summarizes the findings of two longitudinal studies of expert-level cognitive skills and their acquisition. Their empirical, theoretical, and methodological contributions are described and some practical implications of this work are discussed. Comprehensive analyses of subjects' leaning and performance demonstrate that acquired knowledge, rather than exceptional native talents or general aptitudes, constitutes the foundation of expertise in the skills studied. They show that the knowledge structures and processing supporting expertise can be analyzed at a fine grain. Theoretically- motivated analyses reveal (a) that rapid and reliable storage and retrieval of information in LTM appears to be a key general characteristic experts' information-processing, and (b) that acquired memory skills enable experts to use LTM efficiently to reduce the constraints that limited working memory capacity imposes upon performance in complex tasks. The studies also document the efficient information-processing strategies on cognitive skill acquisition and expert performance. Keywords: Expertise, Information-processing capacity, Memory skill, Cognitive skill acquisition, Memory processes, Strategies.