Color image quantization is used to reduce the number of colors in a digital image with a minimal visual distortion. Color quantization can also be defined as a lossy image compression operation. Even though 24-bit graphics hardware is becoming more common, color quantization still maintains its practical value. Due to the complexity of color quantization, preprocessing steps are commonly employed for reducing the data processed by quantization algorithms. One criterion commonly used by quantization algorithms is minimization of the distance between each input color and its representative. Because the first applications of quantization were used to display full-color images on low-cost color output devices, quantization algorithms from the beginning had to face to two constraints. On one hand, the quantized image must be computed at the time the image is displayed. On the other hand, the visual distortion between the original image and the reproduced one has to be as small as possible.