Computer Generated Holography (CGH) is a promising technique for synthesizing 3D images on-demand. CGH systems sculpt light by computing a 2D modulation pattern to shape the phase or the amplitude of a coherent light source. Yet, static wave modulation restricts feasible holograms to a very small subset of all the possible illumination patterns where speckle noise is omnipresent. Here, we introduce Dynamic Computer Generated Holography (DCGH), a novel light sculpting technique that modulates light both spatially and temporally. DCGH has many more degrees of control to eliminate unwanted wave correlations and yields high fidelity, speckle- free 3D holograms. Our technique relies on an algorithm that simultaneously computes a set of engineered coherent wavefronts by optimizing them as a whole to best render the desired 3D illumination pattern when their contributions are rapidly superimposed and averaged in time. We have implemented DCGH with a Digital Micromirror Device (DMD) and synthesized 3D holograms made of up to 50 mutually optimized waves with refresh rates of 190 3D images per second. Our experimental results indicate that DCGH yields 3D images with improved resolution and contrast, successfully addressing the shortcomings of single-frame holography. Our technique can be implemented with inexpensive, low-power hardware, as a compact, portable 3D display that is perfectly suited for virtual and augmented reality applications.