The main objective of this chapter is to present the constitutive equations of a lamina arbitrarily oriented with respect to a reference coordinate system. This is a necessary step before studying the mechanics of the laminate in the next chapter. First, a review of stress and strain is presented. Then, the assumption of plane stress is introduced to obtain a reduced version of the constitutive equations for a lamina. A review of the coordinate transformations precedes a section on off-axis constitutive equations for a lamina. Finally, the three-dimensional constitutive equations are developed.