Affective instability is widely regarded as being the core problem in patients with borderline personality disorder (BPD) and the driving force behind the severe clinical manifestations of BPD symptoms. In ICD-10, BPD is even labelled as emotionally unstable personality disorder. In the last years, the advent of electronic diaries, in combination with sophisticated statistical analyses, enabled studying affective instability in everyday life. Surprisingly, most recent studies using state-of-the-art methodology to assess and model affective instability in BPD failed to show any specificity, supporting the idea of a transdiagnostic construct. In addition, dysfunctional emotion regulation strategies revealed results contradictory to current clinical beliefs. Using multiple data sets and multilevel modelling, we will demonstrate that to understand affective instability it is important: – to statically model basic subcomponents of affective dynamics simultaneously; – in combination with dysfunctional regulation strategies; – cognitive processes in everyday life. Altogether, current research suggests that the dynamics of affective states and their intentional regulation are even more important to psychological health and maladjustment, than the affective states itself. Current initiatives to fundamentally improve psychopathological research are looking at basic physiological processes spanning across disorders. However, these approaches do fall short in understanding human behaviours as dynamical processes that unfold in the broadest setting imaginable – everyday life. Only the combination of basic physiological processes and methods assessing dynamical affective mechanisms in everyday life will enhance our understanding how dysregulations and dysfunctions of fundamental aspects of behaviour cut across traditional disorders.