The article reveals the main content of the constructivist concept of the nature of scientific knowledge and its methodology as an alternative to three traditional epistemological concepts: materialism, empiricism and rationalism (a priori). In contrast to the materialist (including Marxist-Leninist) interpretation of the process of scientific cognition as a reflection of material reality by consciousness, constructivist epistemology proceeds from the fact that any kind of knowledge, including scientific knowledge, is an immanent product of creative activity of consciousness, the content of which is only partially determined by the interaction of consciousness with objective reality. The subjective reality constructed by consciousness is only a possible model of objective reality, which is filled with objective content only after its comparison and identification with a certain area of objective reality. Unlike empiricist (positivist) epistemology, according to which the essence of scientific knowledge consists in generalization of experience data by thinking, constructivism asserts that scientific knowledge is the product of creative thinking activity, to a very weak extent determined by the available sensory experience and the laws of formal logic. In contrast to apriorism, the constructivist concept of scientific cognition denies the presence in consciousness of some inherent immanent (a priori) content. According to constructivism, any product of consciousness as a self-sufficient reality in relation to objective reality is the result only of its constructive-projective activity. The product of such activity is not only scientific knowledge, but also the methodology of its acquisition and justification.