Molecular biology of mTOR (mechanistic target of rapamycin) introduces the structure, function, and regulation of the mTOR complexes, which are essential in integrating both intracellular and extracellular signals in a cell. In this chapter, we explore the structure of the mechanistic target of rapamycin (mTOR) molecule, delving into its role across various functions, and brief signaling pathways as we discuss the more detailed mTOR signaling pathways in the next chapter. While the molecular intricacies of mTOR and its pathways might seem daunting, our goal is to demystify these complexities by explaining the basic key players involved in these molecular interactions, for a better understanding the essentials of mTOR complexes. We hope that this chapter would be more accessible and relevant to a broader audience beyond molecular biologists.