Opsins, light-responsive ion-conducting channels or pumps, are employed for genetically targeted modulation of a cell’s membrane potential. Opsins can be activated by bioluminescent light emitted from luciferases. The most efficient way to deliver luciferase and opsin to cells at a 1:1 ratio and to ensure proximity of the light emitter and light sensor is by tethering the luciferase to the opsin in a luminescent opsin, or luminopsin (LMO). Here, we discuss basic design principles of LMOs, including the options for light-emitting and light-sensing components and strategies for combining them. With the availability of brighter luciferases and more light-sensitive opsins, it will become more feasible to drive opsins with bioluminescence generated from co-expressed light emitters, increasing flexibility and combinatorial specificity. While most applications thus far have been in neuroscience, the technology is ready to be expanded to applications in cell types other than neurons.