Left ventricular dyssynchrony (LVD) is common in heart failure patients with LV systolic dysfunction. Contrary to what it could be expected, LVD is not a stable phenomenon. Various conditions (inducible ischemia, exercise, drug administration) may significantly alter the presence and the magnitude of LVD, which could per se modulate response to treatment for heart failure. LVD can be evaluated using validated Doppler-echocardiographic techniques as tissue Doppler imaging. Exercise and dobutamine stress echocardiography can be used tests to unmask LVD. Changes in LV synchronicity during stress test occur independently of inducible ischemia and irrespective of QRS width. The degree of LVD varies substantially from patient to patient. The dynamic increase in LVD represents a strong contributor: (1) to exercise-induced changes in mitral regurgitation, (2) to limitation of stroke volume adaptation during exercise, and (3) to exertional dyspnea. Whether dynamic LVD might independently affect the outcome has not yet been demonstrated. In the setting of CRT, the assessment of dynamic LVD might help patient selection, predict the magnitude of response, and optimize pacing delivery during exercise. Further longitudinal studies are required to confirm the value of assessing dynamic LVD.