Can the experience of being ostracized – ignored and excluded – lead to people being more open to extremism? In this chapter we review the theoretical basis and experimental evidence for such a connection. According to the temporal need-threat model (Williams, 2009), ostracism is a painful experience that threatens fundamental social needs. Extreme groups have the potential to be powerful sources of inclusion and could therefore address these needs, thereby making them especially attractive to recent targets of ostracism. We also identify a set of factors that is theoretically likely to affect this link and review evidence for the opposite causal path: People are especially likely to ostracize others who belong to extreme groups. Together, this suggests a possible negative cycle in which ostracism may push people toward extreme groups, on which they become more reliant as social contacts outside the group further ostracize them.