We have developed a novel optical coding technology for massively parallel and high-throughput analysis of biological molecules. Its unprecedented multiplexing capability is based on the unique optical properties of semiconductor quantum dots (QDs) and the ability to incorporate multicolor QQs into small polymer beads at precisely controlled ratios. The use of 10 intensity levels and 6 colors could theoretically code one million nucleic acid or protein sequences. Imaging and spectroscopic studies indicate that the QD tagged beads are highly uniform and reproducible, yielding bead identification accuracies as high as 99.99 percent under favorable conditions. DNA hybridization results demonstrate that the coding and target signals can be simultaneously read at the single-bead level. This spectral coding technology is expected to open new opportunities in gene expression studies, high-throughput screening, and medical diagnosis.