There is an urgent need to understand the needs of young people in rapidly developing urban spaces. Long-term studies that enable researchers and clinicians to look at aetiological factors that may affect development are required. The chapter investigates what has changed in the reporting of behavioural symptoms and syndromes in children living in Mexico City. Across all age groups, nervousness, restlessness, inattentiveness, irritability, and explosiveness have significantly increased. The relationship between the environment and psychological symptoms is of interest, especially in preschool children who are likely to be irritable. Depressive syndrome with both irritable and depressed mood as core symptoms show an increasing prevalence from preschool age to late childhood; depressive syndrome with only one core symptom, predominantly irritable mood, shows a consistent increase across all age groups. Explosiveness, possibly associated with brain damage, has significantly increased, most notably in older school-age children. This raises significant questions about the environment.