This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book focuses on territorial development processes associated with border cities. It presents a theoretical framework on the role of border cities in promoting territorial development and planning in border regions. The book introduces a theoretical discussion on the concept of territorial development as well as the main role that border cities have in supporting and implementing positive territorial development processes. It deals with an analysis of the territorial trends of 67 identified border cities located in internal and external border regions of the European Union. The book reviews past and current European policies for border cities. It presents a reflection on US cross-border policies supporting border cities, as well as their space and identity. The book presents a wealth of updated knowledge, based on the analysis of six concrete case studies: border cities from both Europe and North America.