The nucleosome is the fundamental repeating unit of DNA organization in the chromosomes of all eukaryotes and accounts for the two most fundamental levels of chromatin structure (1). Not only do nucleosomes efficiently package DNA, they are also intimately involved in the gene expression mechanisms that allow only selected regions of the vast store of genomic information to be read out as a consequence of signaling processes. These two functions require about 25 million nucleosomes in each human cell nucleus. We have determined the three-dimensional structure of the nucleosome core particle, which comprises an octamer of eight histone proteins and 147 bp of DNA, to a resolution of 2.0Å. At 206 kDa, the nucleosome core particle is the largest and most universal protein/DNA complex solved in atomic detail.