In this chapter, I critically review previous studies related to the yídìng family and studies on English adverbs of modal certainty. This chapter consists of four sections, including this one. In Section 2.2, I review descriptive studies on members of the yídìng family. I discuss their strengths and disadvantages. In Section 2.3, I review studies on English adverbs of modal certainty, such as really, certainly, definitely, etc. I explain why the analyses proposed in these studies cannot be applied to the yídìng family in Chinese. Section 2.4 summarizes this chapter.