Women are under-represented in science, technology, engineering and mathematics (STEM) majors and careers in most industrialized countries around the world. This paper explores the broad array of explanations for the absence of women in STEM put forth in the literature of the last 30 years. It is argued that some proposed explanations are without merit and are in fact dangerous, while others do play a part in a complex interaction of factors. It is suggested that the very nature of science may contribute to the removal of women from the "pipeline". Recommendations for reform in science education to address this problem are also provided.