What is the most important factor influencing human learning? Different instructional theories give very different answers to this question, because they take different perspectives on learning and thus focus on other desired learning outcomes, other methods affecting learning processes, and other conditions under which learning takes place. This chapter describes how eight prevailing research paradigms have influenced and still strongly influence theory development in the field of educational communications and technology. According to the perspective of Gestalt psychology, the most important factor influencing learning is insight and understanding; according to behaviorism and neo-behaviorism, it is reinforcement; according to developmental psychology, it is the learner´s stage of cognitive development; according to cultural-historical theory, it is interaction with the world; according to information processing theory, it is active and deep processing of new information; according to cognitive symbolic theory, it is what the learner already knows; according to cognitive resource models, it is the limited processing capacity of the human mind, and according to social constructivism, it is the social construction of meaning. It is argued that research is typically done within one particular paradigm, but that researchers should be conscious of the fact that paradigms heavily affect their research methods and findings. Moreover, researchers should be open to alternative theories and paradigms because new developments often take place at the interface between paradigms.