The concept of boundary spanning regimes has emerged to describe activity across policy subsystems that seek to manage 'wicked' public policy problems. This paper examines two existing public policy theories, namely those of exogenous shocks and the Advocacy Coalition Framework theories and assesses their capacity to explain why boundary spanning regimes emerge. It argues that broad structural conditions play an important role in shaping boundary spanning activity in the case studies discussed in this paper, indicating limitations in these theories which tend to overlook such conditions. The paper tests the explanations for policy change through original qualitative analysis of incremental convergence across the welfare and immigration policy fields in Australia from 1947 to 1996.