Structures deform not only under loading but also under thermal conditions. Most materials expand with increased temperature. In engineering, this cannot be ignored or it will result in unexpected behavior in the designed structures or products. In some extreme cases, local or global buckling might be triggered because of thermal conditions. Thermal expansion is a material property. Different materials expand to different extents under the same thermal condition. This course defines thermal strain based on the coefficient of thermal expansion. In addition, the relationship between stress and thermal strain is explained with several simulation examples.