Light-emitting diodes (LED) are advancing the development of indoor vertical farming due to their varied light spectrum and photon flux density. The substantial energy cost of lighting systems requires improvement. This chapter reviews three major spectra applied to lettuce cultivation in indoor vertical farming systems. Red and blue light are the most efficient spectra for chlorophyll absorption and far-red light promotes light interception and increases photochemical efficiency. Research shows that the photon flux ratio between red and blue light affects plant architecture, leaf and photosynthetic characteristics, biomass production and phytochemical compound accumulation. Additional far-red over photosynthetically active radiation triggers shade avoidance responses and increases light interception, which combines with planting density effects. Aside from the above spectra, biomass production and quality, utilization of other light spectrum and dynamic lighting as well as plant modelling should be considered to promote light use efficiency in indoor vertical farming.