Biosecurity is a strategic and integrated approach that encompasses the policy and regulatory frameworks that analyse and manage risks to plant and animal life as well as health, including associated environmental risk. It covers all activities aimed at managing the introduction of alien species to a particular region and mitigating their impacts should they become invasive. This includes the regulation of intentional (including illegal) and unintentional introductions and also the management of invasive alien weeds, pests and pathogens by central and local government, industry and other stakeholders. Biosecurity activities occur along a continuum that starts offshore or pre-border in order to reduce the risks posed by other countries, then at a nation’s borders to stop pests, pathogens and weeds from entering a particular region and finally within a region or post-border to locate and eradicate or manage risk organisms that have crossed the border and established in the region. Biosecurity management is a major challenge that requires knowledge and analysis of the diverse and complex risks along this continuum to identify, prioritise and apply measures in a coherent manner to progressively reduce the risks. To illustrate this challenge, both the range of economic and environmental impacts posed by alien species as well as the diversity of pathways with which these species are introduced into a new region are reviewed. Against this background of increasing biological invasions, the effectiveness of current regulatory tools is appraised and forms the basis for recommendations as to where improvements in the management of biosecurity threats should be made. Effective biosecurity is limited by problems in obtaining an objective measure of the hazards posed by alien species, challenges of predicting complex hierarchical and nonlinear systems, difficulties in quantifying uncertainty and variability, as well as cognitive biases in expert judgement. Other approaches include scenario planning that seeks qualitative inputs regarding hypothetical events to facilitate long-range planning using multiple alternatives each explicit in their treatment of uncertainty. This represents a change from prevention towards adaptive management where the difficulty in prediction is acknowledged and investment targets early detection, mitigation and management.