Volunteering as a complement to formal care is important in shaping a participation society and is necessary due to increasing healthcare costs and the reduced availability of healthcare personnel. However, little is known about the mental health and factors that influence the mental health of a specific group of volunteers: crisis line volunteers. Crisis line volunteers offer a listening ear 24/7 and are effective in decreasing feelings of hopelessness and psychological pain and even in preventing suicide for anyone who cannot or does not want to make use of formal care. Although working at crisis line service is rewarding for many volunteers, a number of studies have shown that volunteers can experience distress and compassion fatigue. This thesis therefore examined the mental health of these volunteers and the work characteristics that influence this, with a focus on the role of self-compassion.