The covalent modification of histone tails has emerged as a crucial step in controlling the eucaryotic genomes. Eucaryotic cells must possess mechanisms for condensing and decondensing chromatin. Moreover, chromatin condensation is particularly evident during mitosis and apoptotic cell death, whereas chromatin relaxation is necessary for replication, repair, recombination and transcription. The post-translational modifications of histone tails such as reversible acetylation, phosphorylation and methylation play a critical role in dynamic condensation/relaxation that occurs during the cell cycle. Histone phosphorylation is believed to play a direct role in mitosis, cell death, repair, replication and recombination. In this review, we discuss recent progress in studies of histone phosphorylation.