A spectrum of severity of paranoia (unfounded thoughts that others are deliberately intending to cause harm) exists within the general population. This is unsurprising: deciding whether to trust or mistrust is a vital aspect of human cognition, but accurate judgment of others’ intentions is challenging. The severest form of paranoia is persecutory delusions, when the ideas are held with strong conviction. This paper presents a distillation of a cognitive approach that is being translated into treatment for this major psychiatric problem. Persecutory delusions are viewed as threat beliefs, developed in the context of genetic and environmental risk, and maintained by several psychological processes including excessive worry, low self-confidence, intolerance of anxious affect and other internal anomalous experiences, reasoning biases, and the use of safety-seeking strategies. The clinical implication is that safety has to be relearned, by entering feared situations after reduction of the influence of the maintenance factors. An exciting area of development will be a clinical intervention science of how best to enhance learning of safety to counteract paranoia.