The book examines which linguistic properties are hard and which are easy to acquire when learning a second language. The Bottleneck Hypothesis is proposed, which states that inflectional morphemes and their features present the most formidable challenge to learners, while syntax and phrasal semantics pose less difficulty. The numerous empirical studies reviewed in the book indicate that there is no critical period for acquisition of meaning. The findings suggest that an enhanced focus on practicing grammar in language classrooms will be beneficial to learners.