Quality improvement (QI) approaches are becoming increasingly important in the delivery of mental healthcare internationally. They were originally developed in the manufacturing industry, but the principle of having a systematic approach to improvement has spread to many other industries, not least to healthcare. Quality improvement approaches in healthcare were pioneered in the USA at organisations such as Virginia Mason and the Institute for Healthcare Improvement. In recent years, they have become firmly established in mental health services in the UK's National Health Service (NHS). There are a number of different approaches to quality improvement, but two leading models have taken root: ‘lean thinking’ (also known as ‘lean methodology’ or simply ‘lean’), which arose out of Virginia Mason, and the ‘Model for Improvement’, which came out of the Institute of Healthcare Improvement. This article describes these two quality improvement approaches, critiques their philosophy and explores how they can apply in the provision of mental healthcare, particularly with reference to the use of data, evidence and metrics.