This encyclopedia explores the role of theories in STEM disciplines, examining how they shape understanding and learning in these fields. It delves into the construction, evolution, and significance of theories, highlighting their importance in unlocking the mysteries of the world. The work features over 200 expert-authored articles, organized thematically with a Reader's Guide. Each entry includes further readings, cross-references, and a Resource Guide listing key books, journals, associations, and websites. This comprehensive reference provides valuable insights into the theoretical foundations of science, technology, engineering, and mathematics.