This work describes the effect of microstructure on the behaviour of small fatigue cracks in the two Al-Li-Cu-Mg-Zr alloys 8090 and 8091. The slip distribution in these alloys was varied through thermo-mechanical processing. Although slip distribution affected tensile, cyclic and long fatigue crack behaviour, it did not affect small fatigue crack behaviour due to low levels of closure and reduced slip reversibility near to a free surface. Similarly, underageing and overageing had no effect on small fatigue crack behaviour. Therefore, small fatigue crack growth is concluded to be insensitive to precipitate size and distribution. Due to differences in grain size, small fatique cracks in 8091 were observed to propagate twice as slowly as in 8090. Small fatigue cracks grew more rapidly than long fatigue cracks under the same nominal K and displayed no threshold behaviour. This was shown to be due to the combined effects of the low closure levels, high applied stresses and proximity to the surface. Corrections to K to account for these factors were shown to be less effective than using J as a correlating parameter. None of these factors affected the suitability of K for characterizing microstructural effects. Small and long fatigue crack data were comparable when plotted versus calculated plastic zone size. Low cycle fatigue behaviour was shown to be related to small fatigue crack behaviour and similar microstructural effects were observed. Total rather than plastic strain was identified as the critical parameter when comparing small fatigue crack and LCF behaviour. Small fatigue crack growth behaviour was shown to be inconsistent with the plastic work accumulation criterion for the exhaustion mechanism of fatigue. Finally, because of low K's and the lack of closure, fatigue mechanisms may be easier to study in small cracks than in long cracks. (D91005)