Packaging of the eukaryotic genome into chromatin functions not only to constrain the genome within the boundaries of the cell nucleus but also to permit dynamic and broad-ranging changes related to many important biological phenomena. Therefore, chromatin assembly is a process that affects DNA replication, repair, and gene expression. Chromatin structure is linked to transcriptional regulation, and recent studies show how chromatin is altered so as to facilitate transcription. In addition, modification of chromatin structure is an important regulatory mechanism. Here I review the mechanism of chromatin assembly in vitro and the changes of chromatin structure related to transcriptional activation.