Globular Clusters (GCs) exhibit star-to-star variations in specific elements
(e.g., He, C, N, O, Na, Al) that bare 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, although, to date, has not been found in any massive
cluster younger than 2~Gyr. 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, in
particular when a global view of the GC population is taken. We review the
state of current observations, and outline the successes and failures of each
of the main proposed models. The traditional idea of using the stellar ejecta
from a 1st generation of stars to form a 2nd generation of stars, while
conceptually straight forward, has failed to reproduce an increasing number of
observational constraints. We conclude that the puzzle of multiple populations
remains unsolved, hence alternative theories are needed.Comment: To appear in Annual Reviews of Astronomy and Astrophysics (volume
56). Author's own version. Please see the journal website for the final
published version: http://www.annualreviews.org/journal/astr
