Abstract The Mechanical Specific Energy (MSE) model is used widely in the drilling industry. However, its use has been questioned in a two-part series in the SPE-IADC Drilling Conference, "Mechanical Specific Energy (MSE): Claims and Implications – Facts, Fallacies, and Pitfalls" (Samuel and Mensa-Wilmot, 2023 SPE 212508, 2024 SPE 217727). This paper will independently develop the theory of the MSE, leading to an improved understanding and an appraisal of the conclusions in the two papers. This paper develops the theory underlying the MSE from fundamentals and explains its applications and constraints. The theoretical development uses the concept of instantaneous mechanical power, which it transitions to drilling energy and then to drilling energy per a specific volume of rock, the MSE. The development pays particular attention to physical units. Of special interest is the relationship of the MSE to rock parameters, and its use in defining mechanical efficiency, and limits, of the drill bit. The paper further examines the validity of the Samuel and Mensa-Wilmot claims. The MSE is a valid method for describing the amount of energy delivered by the drilling system to the drill bit to destroy rock. It is calculated as specific to a certain volume of rock, which in physics relates to the strain energy density (SED) of the formation. That is, the concentration of mechanical energy required to catastrophically fail (destroy) a volume of rock. Comparison between the MSE and the SED of the formation describes the mechanical efficiency of the drill bit in converting input mechanical energy to output energy. Hydraulic energy applied at the drilling workface can both improve (e.g., clean, kerf) or impede (e.g., increase pressure overbalance) energy transfer at the workface. It is, therefore, considered separately. Current oilfield measurement capabilities, and theoretical misunderstandings, introduce simplifications and errors in the MSE. It is important to understand this when using the MSE. Several of Samuel's and Mensa-Wilmot's deprecating claims about the MSE are questionable. The paper examines these claims and describes why they are incorrect. However, they do advocate a holistic approach to drilling process management—essentially an operational approach—which has merit. Understanding of the MSE is important in that approach. This paper details the theoretical development of the MSE from a concept of instantaneous drilling power. This places the MSE on a firm theoretical basis and leads to an understanding of its limitations, and valid application. The relationship of the MSE to the energy density of the formation shows that the MSE should not be compared to the compressive strength of the formation, which leads to an improved understanding of drill bit mechanical efficiency.