Tendons, contrary to common thinking, are active and dynamic structures enabling the transmission of tensile forces from muscle to bone. This function depends on the proper structure of the tendon.  There are constant and reciprocal interactions between tendon structure and biochemistry on one side, and tendon mechanical function on the other.  Though type I collagen is the main structural component of tendons and thus it is the final arbitrator of biomechanical parameters, many other molecules are involved in regulation of collagen fibrils and fibers synthesis, assembly and organization. Most of these regulatory proteins are either proteoglycans (such as decorin and biglycan) or glycoproteins (such as tenascin). Others are transcription factors (e.g., scleraxis) and growth factors, most notably, TGFβ. The relationships among these components and their role in normal embryonic development, function and disease are discussed.