This chapter presents the intuition behind why nonignorable nonresponse can be a problem and how it can arise in many contexts. With a foundation that explicitly centers this possibility, we can better reason through when the problem may be larger, how to diagnose it, and how to fix or at least ameliorate it. Section 5.1 describes qualitatively when nonignorable nonresponse may be likely. Section 5.2 works through the intuition about how and why nonignorable nonresponse undermines polling accuracy. Section 5.3 presents a framework for modeling nonignorable nonresponse and culminates by describing Meng’s (2018) model of sampling error. Section 5.4 raises the possibility that nonignorability varies across groups, over time, and even across questions.