This is an introduction to the study of the human Language Making Capacity. More accurately, it is a textbook presenting research questions and research results referring to specific manifestations of this capacity in monolingual and bilingual first language acquisition and child and adult second language acquisition. A more comprehensive treatment of this subject would have to address other aspects as well, for example, the genesis and change of languages, creolization and pidginization, language attrition and loss, impaired acquisition, and so forth. However, such a comprehensive study of the Language Making Capacity is not yet an established discipline within the language sciences. Rather, its manifestations are usually investigated separately, with only limited interactions among the various domains of this field of research. This is true even for closely related domains like first and second language acquisition, or monolingual and bilingual first language acquisition – at best, the mutual interest can be characterized as one-sided, second language research being reasonably well informed about results of investigations into first language development, or bilingual studies about work on monolinguals – but not conversely. Assuming that the various types of acquisition are indeed strongly shaped by the Language Making Capacity – though to different degrees and in distinct ways – this is an unfortunate state of affairs, and I hope with this book to contribute to a change for the better.