The heart of this book is constituted by the scientific discipline of neurolinguistics. The fascinating field of neurolinguistics encompasses the study of the neural mechanisms in the human brain that control the comprehension, production, and acquisition of a native or foreign language. Neurolinguistic research examines the underlying mechanisms of all the major subcategories of language in the brain, e.g. syntax, semantics, phonetics, phonology, or morphology/lexicology, each of which exhibits a distinct neural basis in the brain. Unfortunately, previous research on differences in foreign language (L2) proficiency has concentrated primarily on age-related variables and neglected to examine why there are still striking individual variations even after age-related variables (e.g. age of arrival, length of residence) have been accounted for. Therefore, this thesis analyses the influence of two other major factors, which have frequently been hypothesized to bear on linguistic prowess: musical aptitude and gender. Whereas this paper also addresses the brain-language relationship with regard to native languages, the primary focus of this book clearly lies on the neurological basis of foreign language proficiency. By closely analyzing and interpreting the methodology and results of over 400 peer-reviewed and up-to-date scientific studies published in renowned specialized journals from the field of neurology, the author of this thesis, an interpreter and passionate linguist by profession, strives to elucidate the controversial question of whether neurological and organizational determinants in the brain of any individual, as well as musical talent or age- and gender-related factors, may account for differing levels of foreign language proficiency, native-like pronunciation, and individually distinct recovery rates following brain injuries and strokes.