More than 1 million organisations around the world are certified ISO 9001 and ISO 14001, these standards being based on very similar management practices. Despite the increased number of certified organisations, the main problems and critical success factors in ISO management standard implementation -as described by managers and employees who work with these management systems -have remained largely unexplored. The objective of this paper is to show how ISO management systems can improve in-house practices and avoid the most frequently observed drawbacks. Based on qualitative interviews with 189 managers and employees working in ISO certified organisations, our results showed that the positive or negative impacts of ISO management systems were not a foregone conclusion, but rather depended on specific factors. By shedding light on these factors, the paper offers guidelines in the use of ISO standards that can improve their efficiency and mitigate the risks of improper use. ISO certification should not be considered as a goal in itself, but rather as a learning process with its own pitfalls, benefits and surprises.