Both sexual and asexual fungi undergo chromosomal rearrangements, which are the main cause of karyotype variability among the populations. Different recombination processes can produce chromosomal reorganizations, both during mitosis and meiosis, but other mechanisms operate to limit the extent of the rearrangements; some of these mechanisms, such as the RIP (repeat-induced point mutations) of Neurospora crassa, have been well established for sexual fungi. In laboratory strains, treatments such as mutation and transformation enhance the appearance of chromosomal rearrangements. Different DNA sequences present in fungal genomes are able to promote these reorganizations; some of these sequences are involved in well-regulated processes (e.g., site-specific recombination) but most of them act simply as substrates for recombination events leading to DNA rearrangements. In Penicillium chrysogenum we have found that short specific DNA sequences are involved in tandem reiterations leading to amplification of the cluster of the penicillin biosynthesis genes. In some cases, specific chromosomal rearrangements have been associated with particular phenotypes (as occurs in adaptive-like mutants of Candida albicans and Candida stellatoidea), and they may play a role in genetic variability for environmental adaptation.