This paper presents a comprehensive non-gray theory of the atmospheres, spectra, colors, and evolution of extrasolar giant planets (EGPs) and brown dwarfs for effective temperatures below 1300 K. The theory covers a wide range of mass and age parameters, making it the first spectral study down to 100 K. The authors find that absorption by hydrogen at longer wavelengths and water opacity windows at shorter wavelengths redistribute flux towards the blue. Methane becomes the dominant carbon-bearing molecule below 1200 K, and its presence is a universal diagnostic feature of EGP and brown dwarf spectra. The primary bands to search for are Z (∼1.05 μm), J (∼1.2 μm), H (∼1.6 μm), K (∼2.2 μm), M (∼5 μm), and N (∼10 μm). Enhancements of the emergent flux over blackbody values, particularly in the near-infrared, can be orders of magnitude greater than expected. The infrared colors of EGP and brown dwarfs are much bluer than previously believed, with the K band flux reduced by methane and hydrogen absorption. The study also reveals that for effective temperatures below 1200 K, most or all true metals are sequestered below the photosphere, and an interior radiative zone is a generic feature of substellar objects. Clouds of water and ammonia form for effective temperatures below ∼400 K and ∼200 K, respectively. The detection ranges for brown dwarf/EGP discovery by ground- and space-based telescopes are larger than previously estimated.This paper presents a comprehensive non-gray theory of the atmospheres, spectra, colors, and evolution of extrasolar giant planets (EGPs) and brown dwarfs for effective temperatures below 1300 K. The theory covers a wide range of mass and age parameters, making it the first spectral study down to 100 K. The authors find that absorption by hydrogen at longer wavelengths and water opacity windows at shorter wavelengths redistribute flux towards the blue. Methane becomes the dominant carbon-bearing molecule below 1200 K, and its presence is a universal diagnostic feature of EGP and brown dwarf spectra. The primary bands to search for are Z (∼1.05 μm), J (∼1.2 μm), H (∼1.6 μm), K (∼2.2 μm), M (∼5 μm), and N (∼10 μm). Enhancements of the emergent flux over blackbody values, particularly in the near-infrared, can be orders of magnitude greater than expected. The infrared colors of EGP and brown dwarfs are much bluer than previously believed, with the K band flux reduced by methane and hydrogen absorption. The study also reveals that for effective temperatures below 1200 K, most or all true metals are sequestered below the photosphere, and an interior radiative zone is a generic feature of substellar objects. Clouds of water and ammonia form for effective temperatures below ∼400 K and ∼200 K, respectively. The detection ranges for brown dwarf/EGP discovery by ground- and space-based telescopes are larger than previously estimated.