This introductory chapter provides a definition of multimedia learning, offers a rationale for multimedia learning, outlines the research base for multimedia learning, summarizes changes since the first edition, and draws distinctions between the two approaches to multimedia design, three metaphors of multimedia learning, three kinds of multimedia learning outcomes, and two kinds of active learning. Multimedia represents a potentially powerful learning technology that is, a system for enhancing human learning. A practical goal of research on multimedia is to devise design principles for multimedia presentations. In addressing this goal, it is useful to distinguish between two approaches to multimedia design a technology-centered approach and a learner-centered approach. Response, strengthening, information acquisition, and knowledge construction are three metaphors of multimedia learning. Research on learning shows that meaningful learning depends on the learner's cognitive activity during learning rather than on the learner's behavioral activity during learning.