Most multi-cellular organisms adopt a specific gene expression pattern during cellular differentiation. Once established, this pattern is frequently maintained over several cell divisions despite the fact that the initiating signal is no longer present. Differential packaging into chromatin is one such mechanism that allows fixation of transcriptional activity. Recent genome-wide studies demonstrate that actively transcribed regions are characterized by a specific modification pattern of histones, the main protein component of chromatin. These findings support the hypothesis that a histone code uses histone post-translational modifications to stably inscribe particular chromatin structures into the genome. Experiments on the dynamics of histone modifications reveal a striking kinetic difference between methylation, phosphorylation and acetylation, suggesting different roles of these modifications in epigenetically fixing specific gene expression patterns.