We investigate the Raman spectrum of the low-temperature orthorhombic phase of the organic-inorganic halide perovskite CH3NH3PbI3, by combining first-principles calculations with high-resolution low-temperature Raman measurements. We find good agreement between theory and experiment, and successfully assign each of the Raman peaks to the underlying vibrational modes. In the low-frequency spectral range (below 60 cm􀀀1) we assign the prominent Raman signals at 26, 32, 42 and 49 cm􀀀1 to the Pb-I-Pb bending modes with either Ag or B2g symmetry, and the signal at 58 cm􀀀1 to the librational mode of the organic cation. Owing to their significant intensity, we propose that these peaks can serve as clear markers of the vibrations of the [PbI3]􀀀 network and of the CH3NH+ 3 cations in this perovskite, respectively. In particular, the ratios of the intensities of these peaks might be used to monitor possible deviations from the ideal stoichiometry of CH3NH3PbI3.