The increasing complexity of medical care environment, and medical education and practices made understanding the principles and concepts of training, development, and the related concepts and disciplines necessary for doctors practicing at almost any level in a healthcare system. The aim of this research paper is to identify and describe the essential training knowledge needed by physicians working in the field of professional training and development (Professional trainers) and medical schools' instruction. These essential knowledge should include in the content of any training of the trainer (TOT) courses conducted for physicians.