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This Resource Letter provides a guide to the literature on the theory of anisotropies in the cosmic microwave background. Introduction The cosmic microwave background (CMB) radiation is a relic of a time when the universe was hot and dense, and as such it encodes a wealth of information about the early universe and the formation of the large-scale structure we see in the universe today. The very existence of the CMB is one of the four pillars of the hot big bang cosmology. That the spectrum [7, 8, 9] is the best measured black body spectrum in nature provides stringent constraints on its origin and on any injection of energy at early times [14]. Perhaps the most exciting and active area of CMB research, however, is the study of its anisotropies: the small fluctuations in intensity from point to point across the sky. As we shall discuss below, these anisotropies provide us with a snapshot of the conditions in the universe about 300, 000 years after the big bang, when the universe was a simpler place. This snapshot is both our earliest picture of the universe and an encoding of the initial conditions for structure formation. As a consequence of the hot big bang model, the CMB was predicted for a long time [20, 21]. The history and drama of its discovery [5, 6] is a full story in and of itself, starting points are the " historical " references [3, 4]. For the anisotropies, although several fundamental calculations were done before 1992, it was with the COBE [10] detection in 1992 that interest and activity exploded.