This paper has two objectives: (1) presenting recent advances in personality theory whereby personality traits are conceptualized within a framework that focuses on the dynamic interactions of behaviour, biology, context, and states, and (2) discussing the implications of these developments for measurement and medical selection. We start by presenting evidence that traits are no longer regarded as stable deterministic predictors of behaviour. Instead, traits are found to change across generations, the life span, and in response to environmental contingencies. Thus, there is an urgent need to explore how traits change as function of medical education. Second, drawing on recent theory and research (behavioural reaction norms and the density distribution model) we highlight evidence to show how the expression of trait relevant behaviour is dependent on context, and is distributed with an average (typical behaviour or personality) and a variance (plasticity or adaptability), with traditional personality measure associated with typical responding. Third, we demystify that some traits are better than others showing that so-called "good" traits have a dark-side. Fourth, we show how these developments impact on how personality might be assessed, thereby presenting recent evidence on the use of contextualized personality measures, situational judgment tests, other reports, and implicit measures. Throughout the paper, we outline the key implications of these developments for medical selection practices.