According to usage-based, constructionist accounts the linguistic input in general and input frequencies in particular play an important role in children's language learning. English-speaking children have been shown to be able to learn entirely novel, invented word order constructions from their input. This book aims to extend this line of research to the area of morphology. Two experimental studies investigate German-speaking and English-speaking children's ability to learn novel morphological constructions from the input. The effects of input frequencies on this learning process are examined in detail. A corpus study provides the morphological background data for the invented constructions and presents additional support for frequency effects from naturalistic language learning. By combining two empirical methods and by exploring morphological learning in two different languages this book provides new insights into the cognitive processes that are assumed to be involved in children's language learning and reveals how these processes are affected by different kinds of input frequencies.