Ablation strategies targeting areas of complex fractionated atrial electrograms are not successful for treatment of atrial fibrillation. Fractionation of atrial electrograms may have multiple causes of both pathologic and nonpathologic origin. In order to gain insight into the definitions used for determining areas of fractionation, a literature search was performed using a systematic approach. A PubMed search for studies describing fractionation during human atrial electrophysiologic measurements resulted in 348 articles that were screened for new definitions of fractionation. The 24studies remaining after screening described 11 different visual definitions for fractionation, 3 automated complex fractionated atrial electrogram detection programs, and 7 new parameters for measuring fractionation. Five different definitions for continuous electrical activity were presented. Electrode properties were often not described, and endocardial bipolar recordings in recent studies used electrode diameters ranging from 1 to 8 mm with interelectrode distance of 2-5 mm. In summary, no uniform definition or recording method is used for measuring fractionation of cardiac atrial electrograms. The different electrophysiologic causes of fractionation and the influence of recording device properties on fractionation complicate identification of true pathologic inhomogeneous conduction. The first step in discrimination between origins of fractionation may be accomplished by relating electrogram morphology to spatial patterns of activation. Before revisiting ablation of areas with fractionated electrograms, we need to determine the correct method for identifying pathologic fractionation.