Growth and development are modulated by environmental signals in many organisms. These signals are often perceived at one stage and 'remembered' until later in development. An increasingly well-understood example of this process in plants is provided by vernalization, which refers to the acquisition of the ability to flower after prolonged exposure to cold. In Arabidopsis thaliana, vernalization involves downregulation and epigenetic silencing of the gene encoding the floral repressor FLOWERING LOCUS C (FLC). This epigenetic silencing is quantitative and increases with the duration of exposure to cold. Vernalization involves a Polycomb-based switching mechanism, with localized nucleation of silencing during periods of cold, and spreading of the silencing complex over the whole gene after the exposure to cold. A number of characteristics of vernalization have recently been elaborated on through the use of mathematical modelling. This has revealed the importance of chromatin dynamics for the switching mechanism and has shown that the quantitative nature of the process is due to cell-autonomous switching of an increasing proportion of cells. The principles derived from vernalization are likely to be widely relevant to epigenetic reprogramming in many organisms.