Self-actualisation is a central theme in many accounts of well-being. Yet theories of how self-actualisation is prosecuted and how exactly it leads to well-being are relatively underdeveloped. This paper addresses this shortfall by explicating a novel theory of self-actualisation-the coalescence of being. The theory is founded on insights from existential philosophy, but these are built upon substantially by integrating recent ideas from psychology, notably self-determination theory, self-discrepancy theory and terror-management theory. The central mechanism of coalescence is the individual trying to harmonise their actual-, ideal- and ought-selves. They do this by positing their ideal-self and ought-self as a goal and then living in accordance with this aspiration. When their actual self is disclosed to them in their actions or in the assessment of others they can gauge their progress towards this goal. Success brings positive affect, while failure gives rise to negative affect. Introspecting upon these signals helps the individual to better understand whether their goals are self-congruent or need adjusting. Iterative recalibration of the three self-constructs on the basis of this introspection accelerates the coalescence process. The final part of the paper analyses how coalescence leads to well-being as defined by a range of theories of well-being.