The pyroelectric response and theD*of thin pyroelectric films on, a substrate are calculated as a function of the substrate and film thickness. It is found that for low‐frequency applications the pyroelectric layer should not be made too thin, preferably several micrometers thick, that the substrate should be as thin as possible, and that the product of its specific heat, density, and heat conductivity should be as small as possible. For thick substrates there is an intermediate frequency region where the value ofD*is practically independent of frequency. This intermediate frequency region extends to higher frequencies if the pyroelectric layer is made thinner, butD*decreases with decreasing film thickness in that region.