Background: Although leadership is a topic which has been extensively researched, there is limited literature concerning co-leadership, especially in connection to start-ups. Moreover, as those who are co-leaders in start-ups often also are co-founders and co-owners, it is vital that their co-leadership dynamic is functional as a way of staying in business. Co-leadership is described as a leadership form which has become more commonly used, which further adds urgency of exploring the topic of co-leadership dynamics, and how to make it functional. Purpose: The purpose of this thesis is to explore how a functional co-leadership dynamic can be created between leaders who are simultaneously founders and owners of a start-up. Method: This is a qualitative study, for which is used semi-structured interviews to collect data from 11 co-leaders in 10 companies. The transcribed interviews have been used together with secondary data to point out specific elements that have shown to be important for a functional co-leadership dynamic. Both the themes of co-leadership dynamics and the context of entrepreneurship, foundership and ownership were used to find and highlight these elements. The elements have been discussed in the Analysis and are thereafter presented in a model. Conclusion: Six elements have been pointed out as important for a functional co-leadership dynamic: Collaborative Attitude, Shared Values & Vision, Open & Continuous Communication, Synergy, Learning & Personal Growth, and Trust. These elements are interrelated, and Collaborative Attitude, Shared Values & Vision, Synergy and Trust are also connected to the contextual factors.