DEEPdown is a self-development programme, grounded in the Five-Factor Model (FFM) of personality, and therefore open to both non-clinical and clinical populations. The name “DEEPdown” refers to the theoretical assumption that human-beings possess somewhat enduring personality traits ‘deep-down’, partly owing to genetic factors; however, it is also assumed that the expression of personality is - to some degree - influenced by environmental factors. DEEPdown is a new programme, in early phase development. Its current iteration has seven 90-minute sessions – an introduction and a conclusion session, and one session designated to each of the big five personality traits. The programme principally involves self-report FFM personality assessment with results shared with participants, education about the FFM of personality, exploration of personality expression, and both self-reflective and behavioural tasks – all of which are geared towards the fostering of self-awareness and self-acceptance. Increasing motivation to change certain aspects of personality (e.g. increasing openness to new experiences) has also emerged as a potential outcome in preliminary testing. We have initially trialled DEEPdown in groups, mainly for pragmatic reasons, but one-to-one and computerised formats are also possible. As an open source intervention, DEEPdown is (1) freely usable, modifiable, or empirically testable by any individual without author permission, and (2) completely accessible, with all programme content available on this OSF webpage. This platform also contains an electronic version of the DEEPdown-based thesis covering its development and early testing (Twomey, 2019), and an overview of DEEPdown, as well as a list of potential research questions. While we welcome pilot studies and other investigations, we caution that DEEPdown is currently in early-phase development and is therefore not ready for routine use in clinical settings, for ethical reasons. Carefully monitored research evaluations in clinical settings are welcome, however. Questions? Please email Conal Twomey cdtwomey@tcd.ie We're very open to collaboration!