Heritage languages are those spoken as a first language in immigrant communities where another language is dominant. This book provides a novel approach to heritage language research by focusing on German as it is spoken in a range of German-origin immigrant communities around the world. It demonstrates, using German as a unique example, how a language can develop under the influence of diverse replica languages on the one hand, and different sociolinguistic conditions on the other. It also includes a new theory of language contact, which combines cognitive approaches on multilingual language representation and language processing, with usage-based frameworks. The analyses cover processes of lexical and semantic transfer, morphosyntactic and syntactic changes and pragmatic aspects, and account for the influence of external factors on individual variation. In addition, the book analyzes socio-psychological aspects, namely attitudes towards language and language awareness, and their influence on individual language maintenance.