Globular clusters (GCs) exhibit star-to-star variations in specific elements (e.g., He, C, N, O, Na, Al) that bear the hallmark of high-temperature H-burning. These abundance variations can be observed spectroscopically and also photometrically, with the appropriate choice of filters, due to the changing of spectral features within the band pass. This phenomenon is observed in nearly all of the ancient GCs, and has recently been found in many younger clusters as well. Many scenarios have been suggested to explain this phenomenon, with most invoking multiple epochs of star formation within the cluster; however, all have failed to reproduce various key observations. I will review the state of current observations and outline the successes and failures of some of the main proposed models. The traditional idea of using the stellar ejecta from a first generation of stars to form a second generation of stars, while conceptually straightforward, has failed to reproduce an increasing number of observational constraints. I conclude that the puzzle of multiple populations remains unsolved, hence alternative theories are needed, and will present new HST and VLT/MUSE results that suggest that we may be finally closing in on origin of this enigmatic phenomenon.