Metal-organic frameworks (MOFs) have garnered considerable attention as exceedingly promising materials recently due to their enduring porosity and adaptable host-guest interactions. Over the past few decades, extensive investigations have been dedicated to the synthesis and diverse applications of MOF materials, encompassing, yet not restricted to, gas storage and separation, sensing, and catalysis. In addition, photoluminescence in MOFs has emerged as a significant area of interest. Herein, we provide an in-depth insight of the origins and fundamentals of MOF photoluminescence, which include basic concepts of photoluminescence and main mechanisms that depict the photoluminescence in MOFs (ligand-centered, metalcentered, ligand-to-metal charge transfer, metal-to-ligand charge transfer, and guest induced). Besides, our review also delineates a perspective to design MOFs and their composites by tuning fluorescent ligands and metal centers, and incorporating fluorescent guest molecules, which are highly beneficial for sensing, white-light emission, optical information protection, and so forth. In addition, the current challenges and promising research directions are pinpointed.