Adverse childhood experiences (ACEs) have been associated with a range of physical and mental health problems, and it is now understood that the developmental timing of ACEs may be critically important. Despite this, there is a distinct lack of methods for the efficient assessment of such timing in research and clinical settings. We report on the development and validation of a new measure, the Adverse Life Experiences Scale (ALES), that indexes such developmental timing within a format incorporating caregivers' reports of ACEs in their own lives and those of their children. Participants were a nationally representative sample of Australian families (n = 515; Study 1), and a sample of clinic-referred families (n = 168; Study 2). Results supported the internal consistency and test-retest reliability of the ALES and indicated high levels of acceptability for the measure. In terms of validity, ALES scores were significantly associated with interview-based measures of child maltreatment and quality of the family environment, as well as measures of psychopathology across multiple informants (parents, teachers, clinician-rated). Furthermore, indices of ACEs occurring within specific age-based periods of childhood were found to explain unique variance in current symptoms of child and caregiver psychopathology, independent of the overall chronicity of those ACEs and current adversity. Public Significance Statement The collection of valid and reliable data on adverse childhood experiences (ACEs) is important to research and clinical practice in a range of settings. The Adverse Life Experiences Scale is a new measure of ACEs that is designed to index the occurrence and developmental timing of such experiences, both in the lives of parents and their children. This is important because existing measures have typically neglected such timing, while focusing on parents and children in isolation.