BioLuminescent OptoGenetics (BL-OG) drives optogenetic elements with internal, locally generated, bioluminescent light created by a nearby luciferase in the presence of its luciferin. This introductory chapter to the primer on BL-OG provides a brief overview of the key elements involved, i.e., luciferases, luciferins, and photoreceptors, the mechanisms of bioluminescent light emission and sensing, the design principles and parameters for creating BL-OG tools, and the many opportunities to improve molecular tools employing bioluminescence. This is followed by a brief overview of the individual chapters that detail the different ways BL-OG has been utilized and applied to increasingly wider areas of research. This includes bioluminescence driving opsins as well as non-opsin photoreceptors and the various intracellular processes targeted, strategies for the use of bioluminescence for communication between cells and for integration of intracellular processes with cellular outcomes, a review of animal studies applying BL-OG tools, and a description of experiences with using BL-OG as a broad educational opportunity. A final Outlook chapter reflects briefly on where developments and improvements might go and on novel possibilities this technology platform might enable.