Although antiarrhythmic drugs are used frequently in the suppression of symptomatic and life-threatening arrhythmias, adverse effects, such as subjective and end-organ toxicity, negative inotropy, and proarrhythmia, are associated with such treatment. The term proarrhythmia is broadly defined as the aggravation of an existing arrhythmia or the development of a new arrhythmia during antiarrhythmic drug therapy. Although proarrhythmia may be a manifestation of toxic levels of antiarrhythmic drugs, its occurrence may be idiosyncratic and occur early after therapy is instituted. This article defines the various types of proarrhythmia, compares the proarrhythmia potential of various antiarrhythmic drugs, defines high-risk patient subgroups, and details strategies to avoid and treat proarrhythmia.