  A sterile neutrino is a well-motivated and widely studied dark matter
candidate. The most straightforward realization of sterile neutrino dark
matter, through the Dodelson-Widrow mechanism, is now ruled out by a
combination of X-ray and Lyman-\alpha measurements. An alternative production
mechanism that is becoming increasingly popular in the literature is the
freeze-in mechanism, involving frameworks where a feeble coupling to a particle
- usually a scalar beyond the Standard Model - in the thermal bath results in a
gradual accumulation of the sterile neutrino dark matter abundance. This
article reviews the various motivations for realizing such frameworks in the
literature, their common characteristic features, and phenomenological
signatures.
