Care planning has been described as a “better conversation” that helps people with long-term conditions to be in control of planning their care. Each person with long-term conditions faces individual challenges and each health care setting is fundamentally different, so there is a need for empirical testing of the specific mechanisms through which care planning may lead to health improvements. A rapid realist review was conducted to unearth underpinning mechanisms leading to outcomes in particular contexts. These are expressed in the form of realist theories, which are developed and refined through the review process. Fifty-one full text studies were included in the review. Seven program theories were iteratively tested and refined. A detailed description of what care planning is and what it should look like in practice has been achieved in the form of realist theories.