A phenomenological treatment of the thermal expansion of a monoclinic crystal is presented. The components of the thermal expansion tensor are expressed in differential form as functions of the unit cell parameters. Two sets of expressions are developed from the Lagrangian and the Eulerian definitions of deformation. These correspond in form to the “instantaneous linear thermal expansion coefficient” and the “true linear thermal expansion coefficient” discussed by Kempter and Elliott (1959). The relation of these forms to the Ohashi and Burnham (1973) equations is also discussed.