Growing numbers of preschool children are being referred to mental health services with symptoms of Attention Deficit Hyperactivity Disorder (ADIID), and medication is increasingly being used to treat them (Zito, Safer, dosReis et al., 2000). However, there is limited evidence for the short- and long- term efficacy of medication (Greenhill, Kollins. Abikoff et al., 2006), concerns over adverse effects (Wigal, Greenhill, Chuang, et al., 2006) as well as ethical objections to medical intervention at such an early stage in development (Perring, 1997). Consequently, there is a need to provide effective psychosocial interventions for this population (Conners, March, Frances, Wells, & Ross, 2001). A literature review of preschool ADHD (Chapter 2) identified three parent-training (PT) programmes that have demonstrated effectiveness in reducing ADHD symptoms, and concluded that PT may provide an effective first-line intervention for preschoolers with ADHD symptoms. The three empirical studies (Chapters 3,4, and 5) evaluated the efficacy of one of these programmes - the Incredible Years (I Y) BASIC PT programme - within a community sample of preschool children with signs of early emerging conduct problems and ADHD. Study one (Chapter 3) examined the short-term efficacy of the lY intervention, using a randomised controlled group design. Following the delivery of the programme, the intervention group was associated with significantly lower levels of parent-reported ADHD symptoms, compared to a wait-list control group. The second empirical study (Chapter 4) examined the longitudinal stability of intervention-Mated gains. Results demonstrated that improvements were maintained at 12- and 18- month follow-ups. Early Intervention for Preschool ADI ID and Conduct Problems xv The third study (Chapter 5) explored potential predictors, moderators, and mediators of outcome within this sample. Moderator analyses failed to uncover significant associations between risk factors and outcome, suggesting that the intervention was effective even for the most disadvantaged families. Predictor analysis suggested that those with the most severe levels of ADHD at baseline were most likely to benefit from intervention. Maternal depression was associated with poorer long-term outcome at 18-month follow-up, although it was not associated with initial treatment response. Mediator analyses indicated that enhanced positive parenting skill was a significant partial mediator of outcome, suggesting that positive parenting is one of the key mechanisms contributing to intervention success. The short-term efficacy, longitudinal stability, and lack of outcome moderators suggest that the lY is a valuable and stable intervention when delivered as a preventive intervention for preschool children presenting with ADHD symptoms, even for families traditionally considered to be 'hard to treat'.