Linguistics, like any discipline, is full of boundaries. However, in nature, as Ruqaiya Hasan points out, there are no clear cut boundaries. The participants of the 42nd International Systemic Functional Congress held at RWTH Aachen University addressed and challenged the notion of boundaries in linguistics in many creative ways. Twenty-one of the papers presented at the congress are collated in this volume. The six sections cover topics that challenge theoretical notions and stances, and explore historical, interpersonal and lexicogrammatical boundaries as well as those between languages and in language development. The volume presents a state of the art overview of systemic functional linguistic theorising with extensions into other theoretical frameworks.