The influence of molecular structure and microstructure on the fracture of high density polyethylene materials has been studied. By changing the chromium-based catalyst composition and optimising polymerisation conditions, polymers having differing levels of branching, comonomer unit placement and breadth of molecular weight (MW) distribution have been prepared. Structural analyses obtained by gel permeation chromatography (GPC), thermal analysis and elution fractionation were correlated with mechanical characteristics obtained from S4 tests, notch pipe tests and modified plane strain impact Charpy tests. The structural factors underlying this correlation are discussed. Specifically, the ability to place short chain branches into high MW fractions appears to provide good resistance to slow crack growth, whereas the degree of crystallinity and lamellar thickness play a decisive role in the rapid crack propagation process. Polymer grades with improved resistance to both rapid crack propagation and slow crack growth have been successfully produced.