This PhD thesis aims to understand the secondary harms of parental substance use (alcohol and illicit drug use) on children’s school outcomes (grades, attendance and behaviour). Parental substance use has deleterious effects on children's health i.e. increased risk of mental health illness, aggression, anti-social behaviour. Children whose parent(s) use substances are at a higher risk of poorer educational outcomes – however, more robust evidence is needed. Alongside this, the research has focused on the directional relationship, and so far, we are unaware of the mechanisms in this relationship. Evidence suggests that parental substance use changes the behaviour of the parent and disrupts the family environment; this can often be compromised of high parental conflict, increased risk of child abuse, ineffective parenting behaviours and low family routine – all aspects which are crucial for children’s educational success. Therefore, this thesis aims to understand the relationship between parental substance use and children's school outcomes and test whether the family-related variables can mediate/explain this.