Numerous delusions have been studied which are highly specific and which can present in isolation in people whose beliefs are otherwise entirely unremarkable - "monothematic delusions" such as Capgras or Cotard delusions. We review such delusions and summarize our 2-factor theory of delusional belief which seeks to explain what causes these delusional beliefs to arise initially and what prevents them being rejected after they have arisen. Although these delusions can occur in the absence of other symptoms, they can also occur in the context of schizophrenia, when they are likely to be accompanied by other delusions and hallucinations. We propose that the 2-factor account of particular delusions like Capgras and Cotard still applies even when these delusions occur in the context of schizophrenia rather than occurring in isolation.