In this review, we report several rational designs of nanowire-based solar cells from single nanowire to nanowire arrays. Two methods of nanowires fabrication: via 'top-down' and 'bottom-up', and two types of configurations including axial and radial junction are presented for nanowire-based solar cells. To enhance absorption, several photon management schemes are shown in detail, including anti-reflection coating, diffractive grating, and plasmonics. Considering the rational design of nanowire arrays, we summarize a total of seven solar cell structures including axial junctions, radial junctions, substrate interfacial junctions, planar junctions, conductors, junctionless and tandem. Each type is supported by examples which are presented and discussed. Finally, a general comparison between bulk and nanowire solar cell efficiencies is given.