Left or biventricular (BiV) pacing, or cardiac resynchronization therapy, was proposed nearly 10 years ago as an adjunctive treatment for patients with advanced heart failure (HF) complicated by discoordinate contraction due to intraventricular conduction delay. Since then, both short-term and a growing number of long-term clinical trials have reported on the mechanisms and short- and mid-term efficacy of this approach, with encouraging results. Therapy is implemented with novel pacing systems incorporating an endocardial lead to stimulate the lateral free wall via a cardiac vein, and often a right ventricular (RV) apex lead to provide BiV stimulation. A third atrial sensing lead monitors intrinsic rhythm and provides timing data to ensure ventricular pre-excitation. Modulation of the electronic atrial-ventricular (AV) time delay can optimize contractile synchrony, enhance the contribution of atrial systole, and reduce mitral regurgitation. Individuals with advanced HF, a wide QRS complex often with an AV time delay, and evidence of contraction dyssynchrony in viable myocardium represent the target patient group. Short-term studies reveal systolic augmentation and chamber efficiency from pacing resynchronization that can be substantial. Long-term studies reveal improved symptoms and exercise capacity, and some report reversal of chronic cardiac dilation. However, important questions regarding long-term efficacy and mortality impact, optimal mode for pacing stimulation, and role of combined pacing/cardioverter/defibrillation devices remain unresolved. Here we review pathophysiologic mechanisms, short- and long-term clinical results, and future directions of this new and promising therapy.