Women's recruitment to ministerial office is improving, but our understanding of what determines women's access to cabinet remains underdeveloped. Comparative studies explain women's ministerial presence through correlations with country-level socio-economic or political factors. This research uses a feminist institutionalist approach to explain gendered access to cabinets. It uses original data from interviews with former ministers and special advisors to map the rules in use of ministerial recruitment. It demonstrates that that women's access to ministerial office is facilitated and constrained by three complexes of predominantly informal rules which structure: who is eligible to be chosen (the eligibility pool); how to qualify (specialist or generalist principles); and who selects ministers (the prime minister or the party). By examining the rules of ministerial recruitment in two Westminster democracies, Australia and the UK, the research demonstrates the differences that exist, even in most similar cases, at system level, at party levels, and over time. By identifying the specific complexes of rules which shape women's access it is possible to identify the rule changes which have been effective in improving women's ministerial opportunities and suggest future reform strategies which would be most effective in the specific context.