Cognitive psychology has focussed mainly on conscious and strategic information processing, but there are many examples for which knowledge is obtained unconsciously and without strategy. Studies on implicit learning show that feature correlations in an environment are attained without conscious effort through examples. This article explains basic experimental paradigms in the study area, and examines factors that affect implicit learning. As learning is affected by study conditions, instructions and structure of stimuli, some effort is needed to integrate the stimuli perceptually for implicit learning. This paper also discusses the abstractness and implicitness of knowledge. Abstractness of knowledge is questionable, but some knowledge over each example is obviously attained. Simulation studies, most of which are based on connectionist approaches, have succeeded in simulating human behavior regarding implicit learning, and so cast doubt on the abstractness of knowledge. Studies about implicit learning provide information about the other side of human cognition. Together with connectionist approaches, they provide valuable knowledge to related areas such as episode-semantic distinction of memory and induction.