This Private Docent thesis proposes a retrospective and integrative perspective on emotional flexibility, reinterpreting five previously published empirical studies through this conceptual lens. While originally designed to address specific questions, these studies collectively address emotional flexibility as a transdiagnostic construct, illuminating different facets of it and highlighting underlying biological mechanisms as well as intervention perspectives. This compilation illustrates how flexible emotional functioning might be enabled, disrupted, and restored by examining its manifestation in diverse populations, such as individuals with Borderline Personality Disorder (BPD), Major Depressive Disorder (MDD), and Traumatic/Acquired Brain Injury (TBI/ABI). The studies employed a range of methodologies such as self-report questionnaires, salivary hormonal assessment, and different interventions, from classical approaches like psychotherapy to more innovative ones, such as neuromodulation and psychedelic-assisted psychotherapy. The integrative discussion addresses themes like the bidirectional relationship between neurobiology and intervention, the potential of transferring knowledge from one population to another, and the thesis limitations. Finally, future perspectives are discussed, with two projects for which grant proposals were submitted. These projects tackle the biological underpinnings of emotional flexibility (such as salivary oxytocin and neural circuits) and its modulation by serotoninergic psychedelic interventions to ultimately improve mood.