The number of quark color charges in QCD is three - which can hardly be considered as a "large" number. However, it has been known since the 1970's that generalizations of QCD, in which the number of colors N is taken to be infinite, are characterized by interesting properties, and offer a neat qualitative explanation for several phenomena observed in the hadronic world. In addition, they also hint at a possible duality between gauge and string theories. During the 1990's, this idea was formulated in a more precise way through the holographic correspondence, which suggests that strongly coupled large-N gauge theories can be described by the classical gravity limit of string models defined in a curved, higher-dimensional spacetime. In this talk, after a concise introduction to these topics, I will present a review of recent lattice studies of gauge theories with more than three colors, and discuss the relevance of the large-N limit for real-world QCD.