The Mayn objective of this work is to analyse the influence of non-associativity and non-coaxiality (irregularity of yield or potential surfaces) on the strain localization modes in the framework of Rice's theory of bifurcation. The theoretical aspects of strain localization are recalled. Two kinds of material are studied: an overconsolidated clay and a sandstone. In the case of overconsolidated clay which is contractant, non-associativity of the plastic law is found to be sufficient for the capturing of localisation in the hardening regime. However, for a strong dilatant material such as Fontainebleau sandstone, non-associativity is no longer sufficient. Non-coaxial flow theory needs to be incorporated in the constitutive law to describe the bifurcation. Comparisons between numerical predictions and experimental data are provided.