The present contribution proposes to transfer the main principles of open source software development to a new context: conceptual modelling; an activity closely related to software development. The goal of the proposed “open model” approach is to collaboratively develop reference models for everyone to copy, use and refine in a public process. We briefly introduce conceptual modelling and reference models, discuss the cornerstones of an open modelling process, and propose a procedure for initiating, growing and sustaining an open model project. The paper concludes with a discussion of potential benefits and pitfalls.