Idiopathic left ventricular tachycardia (VT) has been classified into three subgroups according to mechanism: verapamil-sensitive, adenosine-sensitive, and propranolol-sensitive types. VT can be categorized also into left fascicular VT and left outflow tract VT. Although the mechanism of fascicular VT is verapamil-sensitive reentry, the mechanism of left outflow tract VT is not homogeneous. Fascicular VT can be classified into three subtypes: (1) left posterior fascicular VT with a right bundle branch block (RBBB) and superior axis configuration (common form); (2) left anterior fascicular VT with RBBB and right-axis deviation configuration (uncommon form); and (3) upper septal fascicular VT with a narrow QRS and normal axis configuration (rare form). Posterior and anterior fascicular VT can be successfully ablated at the mid-septum guided by a diastolic Purkinje potential or at the VT exit site guided by a fused presystolic Purkinje potential. Upper septal fascicular VT also can be ablated at the site indicated by a diastolic Purkinje potential. The mechanism of left ventricular outflow tract VT is most likely adenosine-sensitive triggered activity. This VT can be classified into three subtypes according to the location where catheter ablation is successful, i.e., (1) endocardial origin; (2) coronary cusp origin; and (3) epicardial origin. The R-wave duration and R/S-wave amplitude in V1/V2 can be used to differentiate coronary cusp VT from other types of outflow tract VT. Recognition of the characteristics of the various forms of this group of arrhythmias should facilitate appropriate diagnosis and therapy.