Details of indentation-induced mechanical deformation of GaAs, InP and GaN have been studied. In particular, the origin of the discontinuities in the load-penetration curves during loading (so-called 'pop-in' events) was examined. Cross-sectional transmission electron microscopy (XTEM) samples of indents were prepared using focused-ion-beam milling. Atomic force microscopy (AFM) was used to examine the surface deformation after indentation. In all materials, slip appeared to be the prime mechanism of plastic deformation, and, in contrast with Si, no evidence of pressure-induced phase changes was found. Slip along the 111 planes is clearly observed by XTEM and AFM in both GaAs and InP following indentation above the 'pop-in' threshold. At high loads, subsurface median cracking is also revealed in these materials. This cracking appeared to be nucleated at the intersection of the slip planes. This suggests that dislocation pile-up at the slip band intersection and the consequential shear stress build-up cause the nucleation of a microcrack. In contrast, although slip is observed in GaN (predominantly along the basal planes parallel to the surface), no cracking or film delamination has been found. The difference between the crystallographic structures and dislocation densities of wurtzite GaN and cubic GaAs and InP can account for the different deformation modes.