This chapter discusses the individual-level and societal-level factors that underlie why people believe and share misinformation, including analytical and open-minded thinking, political partisanship, trust, political and affective polarization, psychological appeal, repetition, emotion, and intergroup sentiment. We look at misinformation belief and sharing separately: just because someone believes a false claim to be true doesn’t mean they’ll share it on social media, and someone sharing a piece of misinformation doesn’t always mean they also believe it.