Vicarious stress occurs when traumatic events are observed rather than being directly experienced. The outcomes of vicarious stress can result in a higher incidence of negative mental health outcomes that include post-traumatic stress disorder, depression, and anxiety. Vicarious stress requires empathy to facilitate the understanding of other individuals' trauma and suffering, and this is influenced by factors such as familiarity and similar past experiences. To study vicarious stress in rats and mice, a rudimentary form of empathy called emotional contagion, enables these animals to mirror the emotional states of each other, particularly during distress. This requires one animal, the witness, to observe a conspecific endure stressors, from a place of safety. The study of vicarious stress in rats and mice during the juvenile period is sparce, though this critical period of development is vulnerable to stressors that can lead to long-term effects like increased anxiety- and depressive-like behaviours. This thesis furthers the preliminary investigation on how vicarious stress can be studied in juvenile animals and the long-term behavioural changes following these stressors. Considering this it is hypothesized that vicarious stress experienced during the juvenile period will elicit emotional contagion, leading to a sensitized HPA-axis and long-term deficits on exploratory and anxiety-like behaviours in adulthood. Two models of vicarious stress during the juvenile period were implemented, a novel model modified from an existing juvenile stressor model (Chapter 2), and observational fear-learning, an established model implemented in adult rodents (Chapter 3). In adulthood (Chapter 2) and in early adolescence (Chapter 3), the long-term behavioural impacts on exploration, social interactions and fear expression and the HPA-axis reactivity were determined. In Chapter 3, age differences between the juveniles and adults in male and female rats and the impact of past stressor experience in witnesses were explored. The findings revealed that vicarious stress does occur in juvenile rats, and it is not dependent on familiarity, yet it does require past stressor experiences for full behavioral display. Importantly, the extent of emotional contagion, confirmed by fear expression, is lower in male juveniles than it is in male adult rats.