This chapter is very important for the design of beams which carry loads, acting transversely to them. These structures are of importance in the design of buildings, bridges, cranes, ships, aircraft, automobiles and so on. The chapter commences with defining the moment of a force, and then using equilibrium considerations, demonstrates the principle of moments. This is a central skill widely used in designing structures. Several examples are given, with increasing complexity to help the reader acquire these valuable skills. This chapter describes skills which are fundamental and extremely important in many branches of engineering.