Conservation practitioners are increasingly interested in the lessons gained through failure. While other sectors have made significant progress in learning from failure, there is currently limited consensus on how a similar transition could best be achieved in conservation, and what is required to facilitate this. One of the key enabling conditions for other sectors is a widely accepted and standardized classification system for identifying and analyzing root causes of failure. In this paper, we propose a comprehensive taxonomy of root causes of failure affecting conservation projects. To develop this, we solicited examples of real-life conservation efforts that were deemed to have failed in some way, identified their underlying root causes of failure and used these to develop a generic, three-tier taxonomy of the ways in which projects fail. We subsequently tested the taxonomy by asking conservation practitioners to use it to classify the causes of failure for conservation efforts they had been involved in. No significant gaps or redundancies were identified during this testing phase. We then analyzed the frequency that particular root causes were encountered by projects within this test sample, which suggested that some root causes may be more likely to be reported in projects implementing particular types of conservation action, while others may frequently occur across a range of different project types. We propose that this taxonomy could be used to help improve identification, analysis and subsequent learning from failed conservation efforts, address some of the barriers that currently limit the ability of conservation practitioners to learn from failure, and contribute to establishing an effective culture of learning from failure within conservation. This article is protected by copyright. All rights reserved.