How does knowledge of a first or second language develop, and how is that knowledge used in real time comprehension and production of one or two languages? Language development and processing are the central topics that this book explores, initially in terms of first language(s) and then in terms of additional languages. Human growth and development necessarily involve the passage of time, implicating this orthogonal factor and leading to the observation that capacities may vary across the lifespan. Two theoretical frameworks have historically attributed explanations for knowledge and use of language, nature versus nurture approaches: the former credits biogenetic intrinsic characteristics, while the latter ascribes environmental extrinsic experiences as the causes of developmental change. The evidence examined throughout this book offers a more nuanced and complex view, eschewing dichotomy and favoring a hybrid approach that takes into account a range of internal and external influences.