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This volume contains papers presented at the Fourth International Conference on Comparative Biology of the Monocotyledons (Monocots IV) and the Fifth International Symposium on Grass Systematics and Evolution held in Copenhagen in 2008. The previous conferences on monocots in this series were held in 1993, 1998 and 2003, with the idea that an interval of 5 years would allow for research to make significant steps forward between conferences, and a glance at the index of the proceedings of the first conference in the series (held at Kew) reminds me how far our ideas about monocot systematics have moved in the intervening years. In 1993, molecular phylogenetics was still in its (relative) infancy and many of the papers in the proceedings (Rudall et al., 1995) reflect the absence of the phylogenetic analyses that we now take for granted. Composition of some of the major orders and families of monocots was still not resolved, and one of the few papers based on molecular data (Chase et al., 1995, based on rbcL sequences alone) provided evidence for many of these not being monophyletic, as circumscribed at that time. Moving forward to the proceedings of the Third International Conference (Monocots III; Columbus et al., 2006a, b), many of the papers were based on molecular phylogenetic studies, often with data for multiple loci. The trees presented in these studies had greater taxon sampling, and support for the topologies was considerably greater in many cases than those in earlier studies. Monocot studies had moved to a situation in which phylogenetic hypotheses were becoming evermore stable. Comparing the proceedings of Monocots III and Monocots IV reveals clear differences in terms of the subject areas. In Monocots III there were, for example, four general phylogenetic papers on monocots, 12 on Asparagales, five on Liliales, plus others on other monocot groups. In contrast, there are no phylogenetic papers on the monocots in general or on Asparagales or Liliales in Monocots IV; in fact there is only one phylogenetic study of an order (Dioscoreales) in this volume. This clearly reflects the increased knowledge of relationships among and between the major groups of monocots. The authors of papers in the latest volume were able to ask questions on the basis of more-or-less established phylogenetic trees for the larger groups, and the volume has, as a result, a rather different flavour, with most of the phylogenetic studies focusing on smaller taxonomic groups with much more detailed taxon sampling (e.g. in several clades of Poaceae). Comprising 32 papers, this volume represents the state of the art of monocot studies in 2008. Subjects covered, in addition to phylogenetics, include mycoheterotrophy, floral anatomy, pollination, epigenetics, drivers of diversification, complete plastid genomes, fossils and dating of phylogenetic trees. As with the earlier volumes, the editors have brought together an important range of papers written by the experts in their fields. It will be an important volume for years to come and should be given a home in all significant botanical libraries.