As the size of mechanical devices decreases and their power-handling specifications increase, thermal effects will become more and more important. This article is a basic survey of some of the most commonly seen thermal effects in micromechanical optics. The fundamental heat transfer mechanisms of conduction, convection, and radiation are quickly reviewed in regard to typical micromirror-type plates, and a simple measurement technique to extract thermal conductance is described. Interface thermal conductance is discussed in the light of recent experimental results on actuated micromechanical structures and squeeze-film theory. A new class of devices with tunable thermal conductance using controlled interface contact is discussed. Of particular interest are thermal IR detectors with extended dynamic range. Thermal expansion deformation is particularly detrimental to optics with two or more thin film layers. This is described in terms of an analytical elastic model, but the limitations of this are discussed in light of recent research. The model leads to a method for controlling thermal deformation, and micromirror devices are shown that are thermally invariant within λ/60 over at least 37°C. Finally, thermal fluctuation noise is described and shown to limit the finesse of high performance micromachined optical cavities and degrade the sensitivity of thermal nanomechanical detectors.