The operating characteristics of the FID are discussed in terms of the mechanisms of ion formation, the equal per carbon response, the collection of ions and the origin of carrier gas effects. The evidence for the CH-O mechanism is reviewed, and, in view of the occurrence of a special mechanism for nitrile compounds, additional mechanisms are presented which may account for part of the ion formation, particularly from C2H2. The conflicting hypotheses of oxidative and reductive determination of equal per carbon response are explored, and it is proposed that this feature arises in the complete conversion of organic eluates to CH3 prior to their entry into the oxidative zone. The available information on ion-molecule reactions in flames is discussed and found to be of dubious relevance to the FID. Ion collection and carrier gas effects are examined but no definitive conclusions on their origins are possible on the basis of present data.