Before designing a coating it is helpful to have some idea of whether the goal of the design is achievable. The ability to estimate performance limits can avoid fruitless design efforts and avoid the neglect of potential performance gains or simplifications. In the case of antireflection coatings, we have collected a broad range of empirical data on optimized designs to provide a formula to estimate what can be expected in typical cases. This is the thrust of this chapter and some information is also provided for estimating the number of layers and other properties of dichroic, bandpass, and blocking filters.