Apparatus for mapping abnormal electrical activity, comprising: a probe which is configured to receive electrical signal data from respective locations in a heart of a living subject; and a processor which is configured: to automatically analyze the signal data to identify complex fractionated electrograms, CFEs, therein, wherein the apparatus is characterized by the processor being configured: to analyze the CFEs so as to identify reentry locations comprised in the respective locations by assigning a window of interest, WOI, to a given CFE, and identifying qualifying deflections within the WOI, the qualifying deflections comprising peaks lying between predefined minimum and maximum voltages, and evaluating a number of the qualifying deflections, NODs; adjusting the number to account for noise in the qualifying deflections; and verifying that the number lies within a predefined range of numerical values; and to display information derived from the identification in relation to a map of the heart. a probe which is configured to receive electrical signal data from respective locations in a heart of a living subject; and a processor which is configured: to automatically analyze the signal data to identify complex fractionated electrograms, CFEs, therein, wherein the apparatus is characterized by the processor being configured: to analyze the CFEs so as to identify reentry locations comprised in the respective locations by assigning a window of interest, WOI, to a given CFE, and identifying qualifying deflections within the WOI, the qualifying deflections comprising peaks lying between predefined minimum and maximum voltages, and evaluating a number of the qualifying deflections, NODs; adjusting the number to account for noise in the qualifying deflections; and verifying that the number lies within a predefined range of numerical values; and to display information derived from the identification in relation to a map of the heart. to automatically analyze the signal data to identify complex fractionated electrograms, CFEs, therein, wherein the apparatus is characterized by the processor being configured: to analyze the CFEs so as to identify reentry locations comprised in the respective locations by assigning a window of interest, WOI, to a given CFE, and identifying qualifying deflections within the WOI, the qualifying deflections comprising peaks lying between predefined minimum and maximum voltages, and evaluating a number of the qualifying deflections, NODs; adjusting the number to account for noise in the qualifying deflections; and verifying that the number lies within a predefined range of numerical values; and to display information derived from the identification in relation to a map of the heart. to analyze the CFEs so as to identify reentry locations comprised in the respective locations by assigning a window of interest, WOI, to a given CFE, and identifying qualifying deflections within the WOI, the qualifying deflections comprising peaks lying between predefined minimum and maximum voltages, and evaluating a number of the qualifying deflections, NODs; adjusting the number to account for noise in the qualifying deflections; and verifying that the number lies within a predefined range of numerical values; and assigning a window of interest, WOI, to a given CFE, and identifying qualifying deflections within the WOI, the qualifying deflections comprising peaks lying between predefined minimum and maximum voltages, and evaluating a number of the qualifying deflections, NODs; adjusting the number to account for noise in the qualifying deflections; and verifying that the number lies within a predefined range of numerical values; and to display information derived from the identification in relation to a map of the heart.