The principal aim of this thesis is to examine how emotion regulation and mindfulness are related to cardiovascular activity and the implications of these relationships for the understanding of aggression. Studies one and two aimed to detail the relationship of mindfulness to psychopathic traits and emotion regulation. Results of these studies collectively imply that mindfulness shares certain features with primary psychopathic traits, including reduced physiological responding to aversive stimuli. Study three aimed to investigate whether slow-paced breathing, associated with an increase in vagal output and thus decrease in heart rate, may exert effects on emotion regulation. Participation in a paced breathing course improved emotion regulation and increased trait mindfulness in a sample of male offenders. Study four aimed to explore how cardiovascular activity and psychopathic traits relate to female perpetrated intimate partner violence. Increased vagal activity, was found to be positively linked to proactive aggression and partner violence. Study five aimed to extend the results of Study four to a sample of male offenders. The results showed that high vagal activity is related to low empathy and good performance on the Stroop task. Collectively, these findings have implications for the use of mindfulness based treatments and the understanding of aggression.