This open access book is the first to offer a systematic overview of the different methods for assessing brain development and a comparative review of how such assessments have already influenced the law. Lawmakers prefer to draw clear distinctions, but biology is characterized by continua: both in terms of how development proceeds within a person and how it differs from other people. However, this does not mean that age limits are arbitrary. This book extends the author's previous research on the Dutch juvenile criminal law, which was founded on the brain development of adolescents and has been in use for more than a decade. The role of age limits in death and life sentences in the US and the new cannabis legislation in Germany are also analyzed in depth. This project combines biological, psychological and social knowledge and puts forward a pragmatic proposal to connect the two fields of brain development and law. It will be of interest to researchers, professionals (e.g. judges, legislators) and students alike.