Explaining why some linguistic features and constructions are easy or difficult to acquire in a second language has become a prominent current concern in generative second language acquisition(SLA)research.Based on a comparison of findings on the L2acquisition of functional morphology,syntax,semantics and pragmatics,the Bottleneck Hypothesis argues that functional morphemes and their features are the bottleneck of L2acquisition;acquisition of universal syntax,semantics and pragmatics flows smoothly(Slabakova 2006,2008,2013).The article surveys experimental studies supporting this view.A pedagogical implication of this model is discussed,namely,that an enhanced focus on practicing grammar in language classrooms is beneficial to learners. Explaining why some linguistic features and constructions are easy or difficult to acquire in a second language has become a prominent current concern in generative second language acquisition (SLA) research. Based on a comparison of findings on the L2acquisition of functional morphology, syntax, semantics and pragmatics, the Bottleneck Hypothesis argues that functional morphemes and their features are the bottleneck of L2 acquisition; acquisition of universal syntax, semantics and pragmatics flows smoothly (Slabakova 2006, 2008, 2013). The article surveys experimental studies supporting this view. A pedagogical implication of this model is discussed, namely, that an enhanced focus on practicing grammar in language classrooms is beneficial to learners.