The XXIVth IUPAC Symposium on Photochemistry was held in the old university city of Coimbra, Portugal from 15 to 20 July 2012, and welcomed 640 participants from 53 countries presenting their research on this important area of chemistry. This series of meetings started in Strasbourg in July 1964 as the International Symposium on Organic Photochemistry, organized by George Hammond and J. Levisalles. Subsequent symposia have seen the meeting expand to embrace all areas of photochemistry. The program topics of the Coimbra symposium ranged from materials aspects of photochemistry through nanostructures and nanomaterials to mechanistic and synthetic aspects of organic photochemistry, photobiology, photomedicine and skin photochemistry, applied photochemistry, and photochemistry and cultural heritage.The symposium had 8 plenary lectures, 22 invited lectures, 105 oral communications, and more than 400 posters, confirming the vitality of this area of chemistry. It is difficult to pinpoint specific highlights, as these depend very much on one's personal interests, but one of the most important presentations was undoubtedly Tom Meyer's Porter Medal Lecture on metal-to-ligand charge-transfer states in polypyridylruthenium(II) complexes and related systems. An IUPAC Photochemistry Symposium was previously held in Portugal, in Lisbon, in 1986, and it is interesting to note that Prof. Meyer also gave a plenary lecture there addressing some of the fundamental photophysics of these systems [Pure Appl. Chem.58, 1193 (1986)]. It is refreshing to see how these have developed from pure science to practical applications.George Porter gave a plenary lecture at the Lisbon symposium in 1986 on the first nanoseconds of photosynthesis. Developments in instrumentation in the intervening 26 years now make interrogation of excited-state behavior on the femtosecond timescale relatively straightforward, and as various presentations in this volume and in the symposium demonstrate, are helping unravel the importance of early events in many photochemical and photobiological processes.In addition to the lectures and poster presentations, the program also included a number of awards for young photochemists and posters, and a variety of social activities, including canoeing on the local River Mondego.We believe that the scientific program has maintained the excellent tradition of the IUPAC Photochemistry Symposia in showing that this continues to be a vibrant, exciting interdisciplinary area of research. This issue of Pure and Applied Chemistry contains a number of the plenary and invited lectures from the symposium, which we feel mirror the current state of the art of photochemistry as a dynamic and important field of chemistry.Hugh D. BurrowsJ. Sérgio Seixas de MeloArtur J. M. ValenteConference Editors