Purpose: In the 1980s and 1990s enthusiasm for Japanese or "lean" manufacturing methods swept through Western industry. At the time, commentators argued that these methods represented a new paradigm of manufacturing, a radical break with traditional methods. The purpose of this paper is to explore the process of conversion from one paradigm to another, drawing on Kuhn's ideas on the structure of scientific revolutions. Design/methodology/approach: The paper uses a critical-incident approach to illustrate the conversion to, and defence of, a particular view of the world. Two incidents are used. The first is a one-day seminar by a leading proponent in the field, in which the author acted as a participant observer. The second is the response of the UK engineering community to the publication of a report questioning the financial benefits of Japanese or lean manufacturing methods. Findings: Although the introduction of new management methods is typically justified on rational grounds, this paper argues that, in common with the scientific paradigm shifts identified by Kuhn, enthusiasm for lean methods is based on non-rational criteria as well as on their apparently superior efficiency. The language used to discussion of lean ideas in the two critical incidents is reminiscent of that used in religious conversions, and the responses to criticism of the methods are analogous to responses to blasphemy in a religious context. Research limitations/implications: The research is based on analysis of conversion to, and defence of, lean ideas, but it carries implications for many other types of organisational change as well. The findings draw attention to how non-rational criteria can shape the direction of major programmes of change, and hence the direction and strategies of organisations. Practical implications: The paper carries implications for the process by which change can be engendered and managed. It identifies the processes by which conversion to new ideas can occur, identifying critical conditions on the part of the purveyors of the ideas (expertness, trustworthiness and personal dynamism) as well as features of the ideas themselves, such as the availability of local demonstrations of applicability, their aesthetic appeal and their ability to predict events and/or solve problems previously considered to be intractable. Originality/value: The paper represents a novel perspective on processes of organisational change, by likening the process of change to that of scientific revolutions and demonstrating the non-rational aspects of the change process.