Evidence suggests that maternal depressed mood (MDM) and childhood emotional and behavioural disorders (EBDs) frequently co-occur. The aim of this thesis is to investigate the nature of how these psychopathologies develop together across childhood. This epidemiology study uses a large British birth cohort, the Millennium Cohort Study, which charts the development of over 19,000 families throughout the UK. Five subscales of child behaviour were assessed using the preschool Strengths and Difficulties Questionnaire (SDQ) measured at age 3 and the standard school-age SDQ assessed at ages 5, 7, and 11. MDM was measured concurrently using the Kessler 6 scale. A prerequisite stage of analysis involved testing the psychometric invariance properties of the preschool SDQ (Chapter 2). The reliability and construct validity of this measure was established. Measurement invariance across time and predictive criterion validity were demonstrated across preschool to school-age developmental stages. The preschool SDQ was used in conjunction with the school-age SDQ and MDM scales to confirm reciprocity of mother and child behaviours across childhood in Chapter 3. Bidirectional effects were significant and positive across each assessment. The magnitude of effects did not differ by developmental stage, child gender or by agent (mother/child). In Chapter 4, features of change in mother and child behaviours were assessed. Using second order parallel process growth models, initial levels of MDM and all child behaviours were positively and significantly correlated indicating interrelatedness of maternal and child psychopathologies. Child externalizing behaviours at age 3 were significantly negatively correlated with change in MDM over time thus declines in MDM were lower for mothers of children high in externalizing behaviours. MDM at age 3 was significantly, positively correlated with change in peer problems, thus children of mothers with high levels of MDM at age 3 had slower declines in peer problems over time. Results from these studies confirm the interrelatedness of mother and child psychopathologies. Quantitative differences in the relationship between MDM and internalizing compared to MDM and externalizing problems emerged. The practical and clinical implications of these studies are discussed.