‘Exploring the definition of vulnerability’ tackles an important challenge facing our children and families – a challenge that has also engaged the policy community in recent years. This report uses the information from the Growing Up in New Zealand families and children to explore what vulnerability means in the context of the current New Zealand preschool population and our contemporary environment. The approach taken in this report builds on the comprehensive Growing Up in New Zealand cohort descriptions that were provided in three previous reports, Before We Are Born (2010), Now We Are Born (2012), and Now We Are Two (2014). The findings presented in ‘Exploring the definition of vulnerability’ are critical for our children and for our future generations. If we can better determine how to identify those children (and their families) in need of additional support from their earliest days then we can ensure the best possible developmental outcomes and we will be growing a healthy and strong population for all of our futures.The focus of this report is to explore what set of risk factors might be used to define ‘vulnerability’ in the context of New Zealand children and their families. Importantly not all children who are identified as potentially being vulnerable from early life will develop poor developmental outcomes downstream. Understanding what particular characteristics of parents, families and their environments promote resilience in the face of exposure to early vulnerability will also be integral to optimising wellbeing for all New Zealand children, and will be a focus of later reports in this series.