Abstract 1. Introduction 2. Approaches to the Semantics of Yes-No Questions 2.1. The Classical Approach: Denotation Sets 2.2. A Set of True Answers 2.3. partitional Approach 3. A Focus-Sensitive Definition 3.1. Questions in Structured Meaning Approach 3.2. Yes-No Questions in Structured Meaning 4. Default Alternatives 4.1. Default Focus in Declaratives 4.2. Default Alternatives and Yes-No Questions 4.3. '-na' in Korean 5. Summary and Some Residual Problems References