  In Sections 2 and 3 we investigate the circumstances under which it  is plausible to see a Thornian time machine at work. The main  difficulty lies in specifying the conditions needed to make sense of  the notion that the time machine “produces” or is  “responsible for” the appearances of CTCs. We argue that  at present there is no satisfactory resolution of this difficulty  and, thus, that the topic of time machines in a general relativistic  setting is somewhat ill-defined. This fact does not prevent progress  from being made on the topic; for if one’s aim is to establish no-go  results for time machines it suffices to identify necessary  conditions for the operation of a time machine and then to prove,  under suitable hypotheses about what is physically possible, that it  is not physically possible to satisfy said necessary conditions. In  Section 4 we review various no-go results which depend only on  classical general relativity theory. Section 5 surveys results that  appeal to quantum effects. Conclusions are presented in Section 6.