There is a close interrelationship between atrial fibrillation (AF) and atrial flutter (AFL). Atrial fibrillation of variable duration precedes the onset of AFL in almost all instances; during AF, the functional components needed to complete the AFL re-entrant circuit, principally a line of block (LoB) between the vena cavae, are formed; if this LoB does not form, classical AFL does not develop. In contrast, there seems to be a spectrum of atrial re-entrant circuits (drivers) of short cycle lengths (CLs) (i.e., AFL). When the CL of the AFL re-entrant circuit is so short that it will only activate portions of the atria in a 1:1 manner, the rest of the atria will be activated rapidly but irregularly (i.e., via fibrillatory conduction), resulting in AF. In short, there are probably several mechanisms of AF, 1 of which is due to a very rapid AFL causing fibrillatory conduction. All of these interactions of AF and AFL have important clinical implications.