Solar energy represents an abundant (1000 W·m-2) and seemingly cheap source of energy. One way to tap it is to transform light into electricity with photovoltaic devices. Single junction solar cells presently reach 32% conversion yield under 1-sun illumination while multijunction devices irradiated by concentrated light can reach 44%. One major problem encountered when trying to increase the conversion efficiency lies in the spectral mismatch between the absorption spectrum of the semiconductor and the solar emission spectrum. As a remedy, wavelength-converting materials are being developed and because solar cells perform best in a relatively narrow spectral range which depends on their bandgap energy, lanthanide luminescent divalent and trivalent ions are particularly well suited for this purpose. In addition non-luminescent ions feature special crystallographic and conduction properties which make them invaluable in lattice-matched multijunction devices. In this chapter, the performances of rare-earth ions in downconverting, downshifting, and upconverting materials embedded into the architecture of luminescent solar concentrators, silicon solar cells, semiconductor photovoltaic devices, and dye-sensitized solar cells (DSSCs) is comprehensively and critically reviewed. Other contributions, e.g. as scattering layers and as modifiers of the bandgap of titanium oxide in DSSCs, or to the powering of in vivo nanorobots are likewise described. Finally a general assessment is made and improvements on the order of +5 absolute% in conversion yield are predicted provided fully optimized materials can be tailored.