In this book, we present five studies, in which toddlers’ vocabulary and grammar development was assessed using the Slovene adaptation of the MacArthur-Bates Communicative Development Inventories. Within different studies, we analysed vocabulary and grammar development using a longitudinal and a cross-sectional approach; vocabulary and grammar development of boys and girls as well as early language of toddlers, regarding family SES and the frequency of shared reading. In addition, we analysed the agreement between parents’ and preschool teachers’ assessments of toddlers’ vocabulary.