Due to the dominance of the trait approach, personality is widely reduced to a set of trait scores which represent 'usual' behaviour. However, individuals show substantial variation within their personality (e.g. Fleeson, 2001; 2004). Thus there is a need for research into the personality characteristics which underlie this variance so that personality can be more fully quantified. To this end, the current thesis investigated a previously unresearched personality characteristic: personality adaptability, which was defined as: accurate and goal directed selection of personality states across situations which is designed to gain a desired outcomes and which may result in behaviour which is in accordance or discordance with the individual's personal preferences in any given situation. Two studies were run to investigate whether personality adaptability exists as an individual difference. The studies also assessed the validity of personality adaptability by establishing its level of divergence from self-monitoring (Snyder, 1974), and role as a predictor of task performance and satisfaction with life (Diener, Emmons, Larsen, & Griffin, 1985). Study 1 utilised a repeated measures design. Participants completed two tasks, within a laboratory setting, designed to require the opposite poles of normal extraversion. Personality adaptability was quantified by calculating the distance between participants' extraversion level in the two tasks (goal directed state range), and the distance between the required state and observed behaviour in each task (task specific personality adaptability). Within study 2, overt naturalistic observation of stand-up comedians performing stand-up comedy was undertaken. Personality adaptability was represented by calculating the distance between the required state for achieving a successful task outcome and the observed behaviour of each comedian, along comedy relevant facets of personality. Participants of both studies also completed the Satisfaction with Life Scale (Diener et al., 1985) and Self-Monitoring Scale (Snyder, 1974). The study 1 participants also completed a 7-item version of the Big Five Inventory's (BFI) (John & Srivastava, 1999) trait extraversion scale while study 2 participants completed the full BFI. The results of study 1 indicated that, on average, individuals adapted their behaviour in line with the task requirements showing substantially higher levels of extraversion in the high extraversion requiring task, compared to the low extraversion task (d = -1.43, p <0.001). Consistency between personality (extraversion) state was also shown across the tasks (r = .43, p <0.01) implicating trait personality as a determinant of personality state alongside personality adaptability. Both studies indicated personality adaptability to be an individual difference with the factor models extracted, in both studies, indicating a single factor of personality adaptability. Personality adaptability was also shown to be distinct from self-monitoring and trait personality in both studies, and to be the most pervasive predictor of task performance when compared to trait personality and self-monitoring. In study 1, personality adaptability represented by goal directed state range accounted for up to 11% of the variance in the measures of task performance while task specific personality adaptability accounted for up to 47%. In study 2, personality adaptability accounted for up to 41% of the variance in measures of task performance. Self-monitoring and trait personality did not account for any unique variance in task performance within study 2. However, trait extraversion showed a similar effect to goal directed state range on task 1 performance, within study 1 (Beta = .23 and .21, respectively). Personality adaptability was not shown in either study to be a significant predictor of satisfaction with life. Rather the trait personality and self-monitoring factors were the unique predictors of this dependent variable.