  Thin filters and gas tight windows are used in Space to protect sensitive
X-ray detectors from out-of-band electromagnetic radiation, low-energy
particles, and molecular contamination. Though very thin and made of light
materials, filters are not fully transparent to X-rays. For this reason, they
ultimately define the detector quantum efficiency at low energies. In this
chapter, we initially provide a brief overview of filter materials and specific
designs adopted on space experiments with main focus on detectors operating at
the focal plane of grazing incidence X-ray telescopes. We then provide a series
of inputs driving the design and development of filters for high-energy
astrophysics space missions. We begin with the identification of the main
functional goals and requirements driving the preliminary design, and identify
modeling tools and experimental characterization techniques needed to prove the
technology and consolidate the design. Finally, we describe the calibration
activities required to derive the filter response with high accuracy.We
conclude with some hints on materials and technologies presently under
investigation for future X-ray missions.
