A sterile neutrino with a keV-scale mass is a well-motivated dark matter candidate. After an introduction to keV sterile neutrinos and their phenomenology, I will argue that the decay of a scalar boson to sterile neutrinos in the early universe is an attractive possibility to generate the keV sterile neutrino dark matter abundance. I will discuss different scenarios and their implications for the dark matter abundance and the free-streaming horizon which quantifies the scale beyond which sterile neutrino dark matter suppresses the matter power spectrum.