The advent of electrophysiology (EP) testing revolutionized the care of children with arrhythmia. Precise mechanistic and anatomical diagnosis of arrhythmias became possible. The later development of catheter-based ablation transformed the care of these children by allowing many arrhythmias to be cured during the same procedure. Indications for EP testing vary depending on the age of the child, the underlying cardiac anatomy, and the suspected arrhythmia. In the current era, the indications for EPS and for ablation are virtually identical. There are a few situations where EPS is sometimes performed without the use of ablation, and these will be pointed out. This paper will address the common conditions for which EP testing is performed.