A window is used in an optical instrument primarily as a transparent interface between the internal components and the outside environment. Usually, it is a plane-parallel plate of optical glass, fused silica, plastic, or crystalline material that allows the desired radiation to pass through with minimal effect on intensity and image quality, but excludes dirt, moisture, and other contaminants and, in some cases, maintains a positive or negative pressure differential between the internal and external atmospheres. For infrared applications, the window must not radiate due to its temperature in a manner that interferes with the function of the system. Some windows take the form of a deep spherical shell. A deep shell is called a dome. Design and mounting configurations for typical types of windows and domes are considered in this chapter. Optical effects in plane-parallel windows arising from environmental conditions such as temperature and pressure differentials are first discussed. The next topic is the strength of plane-parallel windows. Following are design guides for mounting flat windows. Methods of analyzing dome performance are next. Finally, guidelines for dome mounting are given.