Most of the studies dealing with fatigue crack propagation in elastomers are based either on purely mechanical theories or on fractographic analyses of fracture surfaces. This chapter investigates the fatigue crack growth in both crystallisable and non-crystallisable elastomers under relaxing loading conditions. It focuses on the determination of the mechanism of fatigue crack growth. For this purpose, an original "micro-cutting" method developed for carbon black filled natural rubber (NR) is employed to observe microscopic phenomena involved in the growth of the crack during Scanning Electron Microscopy (SEM) observations. In carbon black filled styrene butadiene rubber (SBR), cavities containing zinc oxides fail when the crack propagates and the heterogeneous deformation only induces filaments which do not resist to crack propagation. Based on SEM observation, a fatigue crack growth mechanism in non-crystallisable SBR is proposed. Results are compared with those obtained with NR.