<p>Environmental conditions, i.e., temperature and moisture, affect mechanical properties of materials, including composites. In this paper, we concentrate on one of the aspects of the effect of environment on fracture in a composite lamina. The paper demonstrates an analytical approach to account for the effect of the changes in environment on the strain energy release rate of composite materials and thin films. These analytically determined strain energy release rates should be compared to fracture toughness to predict the susceptibility of the material to fracture. Numerical examples are presented for several polymeric and metal matrix composites using available experimental data for compliances affected by the environment. While a general conclusion about the effect of the environment on the strain energy release rate of composites cannot be achieved, the materials considered in the examples demonstrated an increase of the strain energy release rate with an elevated temperature or moisture indicating their potential vulnerability to fracture. Additionally, the effect of temperature on the strain energy release rate of thin films delaminated from the substrate is analyzed. The condition of the increase of the strain energy release rate for mode I fracture in a one-dimensional straight-faced blister has been derived. This condition depends on the ratio of the buckling and residual stresses and temperature-dependent moduli of elasticity of the film.</p>