  The low-energy gamma-ray domain is an important window for the study of the
high energy Universe. Here matter can be observed in extreme physical
conditions and during powerful explosive events. However, observing gamma-rays
from faint sources is extremely challenging with current instrumentation. With
techniques used at present collecting more signal requires larger detectors,
leading to an increase in instrumental background. For the leap in sensitivity
that is required for future gamma-ray missions use must be made of flux
concentrating telescopes. Fortunately, gamma-ray optics such as Laue or Fresnel
lenses, based on diffraction, make this possible. Laue lenses work with
moderate focal lengths (tens to a few hundreds of metres), but provide only
rudimentary imaging capabilities. On the other hand, Fresnel lenses offer
extremely good imaging, but with a very small field of view and a requirement
for focal lengths $\sim$10$^8$ m. This chapter presents the basic concepts of
these optics and describes their working principles, their main properties and
some feasibility studies already conducted.
