Histone modifications serve as regulatory marks that are instrumental for the control of transcription and chromatin architecture. Strict regulation of gene expression patterns is crucial during development and differentiation, where diverse cell types evolve from common predecessors. Since the first histone lysine demethylase was discovered in 2004, a number of demethylases have been identified and implicated in the control of gene expression programmes and cell fate decisions. Histone demethylases are now emerging as important players in developmental processes and have been linked to human diseases such as neurological disorders and cancer.