Does government size and structure adapt to changes in government’s organisational environment (particularly to uncertainty and complexity) as predicted by organisational theory? We find – using a range of statistical analyses – support for each of the major theories of organisation adaptation (the contingency-based view, resource-based view, and rational choice view). We find that both government size and structure change – holding other factors constant – for changes in the uncertainty and complexity of governments’ organisational environments. We find seven clusters of governments which adapt their organisational sizes differently in response to changes in the uncertainty and complexity of their organisational environments – and four clusters of governments with differing preferences for the way they adapt governmental structures. We also use the available data to divide governments according to the extent to which they adapt their organisational size and structure reactively (after changes occur in their organisational environment), contemporaneously or strategically (before these changes in their organisational environment occur).