This paper discusses the development of infrared filters of both lowpass and bandpass types for the wavelength region ?>40 µm. The design techniques of multilayer filters have been extended to mesh filters. Bandpass filters at wavelengths near 50 and 100 µm with bandwidths between 5% and 10% and up to 90% transmission have been made using either double-halfwave or Fabry-Perot systems. Lowpass filters of high performance, passband transmission 70% and rejection greater than 103 are shown with edges near 100 µm. Narrowband filters for the HCN laser line at 30 cm-1 are also shown.