This conceptual paper argues that national differences in human resource management (HRM) matter and can be explained by cultural and institutional theories. We explore briefly the nature of each set of theories and some of the critiques that have been made of them. We argue that in general much of the differences in HRM between countries are determined by institutional factors, and that management has more influence over the effect of national cultural differences than it has over institutional differences and therefore in most cases the appropriate tests will be tests against institutional differences. Where institutions are less constraining, cultural differences may be the appropriate template.