The correlation between ultrasonic speckle patterns acquired with different system configurations determines the effectiveness of spatial and frequency compounding, phase aberration correction, elastography, and other applications. For some algorithms, such as compounding, decorrelation has a direct effect on performance. In others decorrelation indirectly degrades performance by reducing the accuracy of signal processing methods such as time delay estimation (TDE). This paper reviews the causes of echo decorrelation for a variety of applications. The mechanisms of decorrelation are described using k-space and simulation. We show examples of signals at different correlation levels to build intuition. The direct impact of decorrelation is described for several of the aforementioned applications. We quantify the indirect effect of decorrelation by examining TDE performance. This analysis shows that reducing correlation from 1.0 to 0.99 increases TDE errors by a factor of 3 under reasonable conditions. This paper also describes a new method of computing speckle pattern correlation directly from system point spread functions, assuming either shift- invariant or shift-variant systems. This paper concludes that echo decorrelation has a major impact on the performance of ultrasound signal processing, and that this decorrelation can be computed using simulation, k-space, or the system point spread function.