The relaxation processes responsible for the finite lifetime of excited vibrational states in molecular crystals are discussed on the basis of recent experimental and theoretical developments. Experimental information on the lifetime of optically active phonons is obtained in the time domain by time-resolved CARS spectroscopy or in the frequency domain by high-resolution infrared and Raman spectroscopy. The experimental results are then interpreted in terms of many-body perturbation techniques or of molecular dynamics simulation.