Twenty years ago, Jo Treweek defined Ecological Impact Assessment  as “The process of identifying, quantifying and evaluating the potential impacts of defined actions on ecosystems or their components”. She explained that the basis of EcIA lies in ecological science, and that it requires rigorous techniques of evaluation so that the implications of predicted outcomes can be assessed. These techniques should inform environmental decision-making and sound environmental management. Following the publication of Treweek’s book, the UK’s institute of Ecology and Environmental Management (IEEM) developed guidance for professional ecologists carrying out EcIA under UK legislation . At about the same time the IAIA produced a Special Publication setting out principles to promote “Biodiversity-inclusive impact assessment” . These two publications made the link between the work of an ecologist and that of the planner, lawyer or decision-maker – how the science should “inform”. They also link “biodiversity” – the variability across the components in the natural environment – with “ecology”, the study of the patterns and processes linking those components. In New Zealand, the word “biodiversity” wasn’t in the Resource Management Act as enacted in 1991. That term didn’t come into common usage until after the release of the Convention on Biological Diversity in 1992 . Rather, the RMA 1991 focussed on ecosystems and ecological values, recognising the importance of understanding the fundamental ecological processes in managing resources. The RMA 1991 placed responsibility for implementing policy around protection of ecological values and assessment of effects on ecosystems with regional and local authorities. For most territorial authorities these were areas for which they had no appropriately trained staff, nor existing approaches or methodologies. While central government focused its advice on the approach to preparing an “Assessment of Environmental Effects” it provided little support or guidance on assessment of ecological impacts or effects. In this guidance vacuum local authorities generally proceeded in isolation; in the absence of a body representing the interests of professional ecologists, there was no structured debate amongst those carrying out or auditing assessments around what might constitute good practice. In 2015 some ecologists in the Environmental Institute of Australia and New Zealand prepared a set of ecological impact assessment guidelines for terrestrial and freshwater ecosystems; following a period of feedback and revision, a 2nd Edition was published in 2018 . As Mark Christensen discusses in his article, these remain only a first step in getting an approach and methodology for EcIA that is accepted by ecologists, understood by planners and lawyers, and recognised by decision-makers, 28 years after the RMA was enacted.