Science teachers need to understand what science is, how students learn, how to teach science effectively, and the rationale for their teaching methods. Along this line, this article discusses constructivist learning theory as an alternative to the traditional pedagogy and the origin of various versions of constructivism. Constructivism is defined and used in a variety of contexts including philosophical constructivism, constructivist research paradigm, sociological constructivism, and educational constructivism. Educational constructivism (or psychological constructivism) can be divided into three distinct versions (i.e., individual, radical, and social constructivism) depending on unique ontological and epistemological beliefs that underlie each version. Each version of educational constructivism supports different conceptions of science teaching and learning that are consistent with its specific ontological and epistemological beliefs. In this article, the main tenets of each version of educational constructivism are examined with regard to ontological beliefs, epistemological commitments, and pedagogical beliefs. In addition, two major criticisms on constructivist pedagogy as well as implications for research methods for each version are also discussed.