A number of fatigue crack propagation laws applied in the study of polymers is described. Consideration of the stress field distribution at the crack tip leads to the application of fracture mechanics. It is shown that a simplified relationship of the form da/dN =Fλα, whereφ is a function ofKIC,Kmax,Kmin andKTH appears to be a convenient expression for cyclic crack growth. The effect of mean stress is more complicated than that in the field of metals, the compressive component of cyclic stress may delay the crack growth. Cyclic tests in tension performed on PMMA and PVC are dependent on ΔK and its mean value,K m . The threshold value,KTH, is also influenced byK m but a more complicated behaviour due to strain rate effects may be observed. Other differences, such as the position of upper and lower transition points and growth rate changes with frequence, are noted. The effect of biaxial cyclic loading of PMMA and PVC plates is compared and some differences highlighted. The results available so far indicate little effect of the crack curving on its growth. However, it is shown that, while the increasing biaxiality can substantially retard the crack growth in PMMA, no such effect was recorded in PVC. Finally, it is shown that at very high stress levels (region III), the cyclic crack growth consists of two propagation modes, namely, a pure cyclic propagation, together with slow growth. At lower stress levels, slow growth disappears and the crack propagates in pure fatigue (region II). In region I, the propagation is very slow, without the usual correspondence between cycles and striations. The results recently obtained on glass reinforced plastics (GRP) are also presented and differences highlighted.