Research in adults suggests that intrusive memories and intrusive thoughts (often referred to as intrusive cognitions) are common in members of the general population and are often seen in clinical disorders. However, little is known about the experience of intrusive cognitions in adolescents, particularly in adolescents with major depressive disorder (MDD) and post-traumatic stress disorder (PTSD). The present study sought to gather fundamental data on these phenomena (i.e., frequency, characteristics and appraisals of intrusive cognitions) in adolescents with MDD and PTSD. Adolescents aged 11-18 with MDD (n = 11), PTSD (n = 13) and a non-clinical control group (n = 25) completed structured interviews concerning their intrusive memories and thoughts. Intrusive thoughts were common in all three groups but were particularly frequently experienced in the MDD group. Intrusive memories were expectedly very common in the PTSD group but also experienced by over half of the adolescents with MDD. Both clinical groups reported more negative emotions in response to their intrusive thoughts or memories and appraised these cognitions more negatively than the non-clinical group. Intrusive memories and thoughts are common experiences in adolescents with MDD and PTSD. Emotions and appraisals relating to these cognitions may be targets for psychological intervention in this age group. However, small sample sizes limit the conclusions that can be drawn. Replication is needed with larger numbers of clinical participants.