The current review and synthesis serves to define and contextualize fear of the unknown relative to related constructs, such as intolerance of uncertainty, and contemporary models of emotion, attachment, and neuroticism. The contemporary models appear to share a common core in underscoring the impor- tance of responses to unknowns. A recent surge in published research has explored the transdiagnostic impact of not knowing on anxiety and related pathologies; as such, there appears to be mounting evidence for fear of the unknown as an important core transdiagnostic construct. The result is a robust foundation for transdiagnostic theoretical and empirical explorations into fearing the unknown and intolerance of uncertainty.