Review of axino dark matter

We review the status of axino dark matter. Two hierarchy problems, the strong CP problem and the gauge hierarchy problem, have led to introducing into particle physics a spontaneously broken global Peccei-Quinn symmetry and a softly broken supersymmetry, respectively. Combining them implies the presence of not only an axion, but also its scalar component, saxion, and their fermionic partner, axino. Among these, the axion and the axino are attractive dark matter candidates. Various possibilities for the axino as dark matter are discussed.