IN recent years, several Australasian jurisdictions have developed method- ologies for regulatory assessment of impacts of development on bio- diversity. Some methodologies are gazetted (NSW Government 2005) and some are policies under legis- lation (Department of Natural Resources and Environment 2002; Department of Environment and Climate Change 2007; Department of the Environment, Water, Heritage and the Arts 2009; Norton 2009). Increasingly, the methodologies aim for no net loss of biodiversity or net environmental gain, or to improve or maintain environmental outcomes (Department of Natural Resources and Environment 2002; NSW Government 2005; Environmental Protection Authority 2006; Webb 2009). Some methodologies are deemed as law, while assessments under other methodologies are subject to court challenges (Meyers 1996; Cabarrus 2009). Assessments using deemed methodologies cannot be challenged in court providing the methodology is accurately followed. Some jurisdictions have deemed methodologies to assess impacts of development on biodiversity in some circumstances, more open (and legally challengeable) pro- visions for biodiversity assessment in other circumstances, and direct Ministerial authority in other situations (NSW Parliament 1979, 1995, 2003). Some methodologies are accompanied by software tools and datasets (NSW Government 2005; Department of Sustainability and Environment 2006; Department of Environment and Climate Change 2009). Most methodologies have been in operation for several years. During this time, much has been learnt about best practice for preparing and implementing methodologies and their accompanying tools and datasets. The purpose of this paper is to provide principles for develop- ing and implementing best practice methodologies for assessing impacts of development on biodiversity, for policy makers and their advisors. The principles apply to assessment of individual development appli- cations and projects, rather than to strategic assessment (see Macintosh 2010), although many are relevant for strategic assessment also. Most of the principles also apply to assess- ment of biodiversity for non-regula- tory purposes, such as incentive schemes.