Does the mother tongue influence the learning behavior of a foreign language? If so, foreign language learners should make different types of errors in the use of the foreign language based on their respective mother tongues. Hungarian learners of German would thus make 'Hungarian' types of errors, while French learners of German would make 'French' types of errors. However, the aim of this book is not to provide a purely theoretical overview of the current state of discussion. Rather, I intend to pursue, discuss, and examine a hypothesis, incorporating empirical research, that has been fiercely debated for decades within foreign language acquisition research and still has its proponents: the contrast and identity hypothesis.