Cardiac resynchronization therapy (CRT) has shown benefits in patients with severe heart failure. The traditional criteria to select patients for CRT (New York Heart Association [NYHA] class III or IV, depressed left ventricular [LV] ejection fraction, and prolonged QRS duration) result in at least 30% of the selected patients with no response to CRT. Recent studies with echocardiography have shown that the presence of LV dyssynchrony is an important predictor for response to CRT. However, the recent report from the predictors of response to cardiac resynchronization therapy (PROSPECT) trial suggested that under "real-world" conditions the current available echocardiography: techniques including tissue Doppler imaging (TDI) and myocardial strain-rate imaging are not ready for routine clinical practice to assess LV dyssynchrony. Phase analysis is a recently developed technique that allows measuring LV dyssynchrony from electrocardiogram (ECG)-gated single photon emission computed tomography (GSPECT) myocardial perfusion imaging (MPI). This technique uses Fourier harmonic functions to approximate regional wall thickening over the cardiac cycle and to calculate regional onset of mechanical contraction (OMC) phases. These OMC phases are obtained three-dimensionally over the entire left ventricle to quantitatively assess the degree of LV dyssynchrony. This technique has been compared to TDI and shown promising results in clinical validations. The advantages of this technique over echocardiography in measuring LV dyssynchrony are its automation, its high repeatability and reproducibility. It can be applied to any conventional GSPECT MPI study with no additional procedure. In this review the phase analysis methodology is described and its up-to-date clinical validations are summarized.