This chapter introduces the principles of virtual work and their special cases, which are useful in finding point displacements and forces as well as for determining continuous solutions of beams and circular plates by variational methods and the finite element method. It also introduces the concepts of work done and energy stored in deformable solids subjected to external forces. Then the related energy principles are introduced and illustrated with the help of extensional deformation of rods or bars. In a deformable body, work is done by externally applied forces and internally generated forces. Strain energy is the internal energy stored in the body due to the deformation of the body, and it is the same as the work done by internal forces in moving through their respective displacements. When a mechanical system experiences such variations in its equilibrium configuration, it is said to undergo virtual displacements.