An Optical Flow technique based on the use of Dynamic Programming has been applied to Particle Image Velocimetry yielding a significant increase in the accuracy and spatial resolution of the velocity field. Results are presented for calibrated synthetic sequences of images and for sequences of real images taken for a thermally driven flow of water with a freezing front. The accuracy remains better than 0.5 pixel/frame for tested two-image sequences and 0.2 pixel/frame for four-image sequences, even with a 10% added noise level and allowing 10% of particles to appear or disappear. A velocity vector is obtained for every pixel of the image.