Mechanical engineering is the application of physical principles to the creation of useful reality for machine design. Such solid models may be used as the basis for finite element analysis (FEA) and / or computational fluid dynamics (CFD) of the design. Through the application of computer-aided manufacturing (CAM), the models may also be used directly by software to create "instructions" for the manufacture of objects represented by the models, through computer numerically-controlled (CNC) machining or other automated processes, without the need for intermediate drawings.

Fundamental subjects of mechanical engineering include: dynamics, statics, strength of materials, heat transfer, fluid dynamics, solid mechanics, control theory, pneumatics, hydraulics, mechatronics, kinematics, and applied thermodynamics. Mechanical engineers are also expected to understand and be able to apply concepts from the chemistry and electrical engineering fields. At the smallest scales, mechanical engineering becomes molecular engineering - one speculative goal of which is to create a molecular assembler to build molecules and materials via mechanosynthesis. For now this goal remains within exploratory engineering, and some consider it science fiction.

Related disciplines include; electrical engineering, industrial engineering, systems engineering, civil engineering, nuclear engineering, aerospace engineering, and other engineering disciplines.