This annual review of X-ray fluorescence covers developments over the period 2003–2004 in instrumentation and detectors, matrix correction and spectrum analysis procedures, X-ray optics and microfluorescence, synchrotron XRF, TXRF, portable XRF and on-line applications, as assessed from the published literature. The review also covers a survey of applications, including sample preparation, geological, environmental, archaeological, forensic, biological, clinical, thin films, chemical state and speciation studies. Of particular interest in this year’s review was further work on the development of silicon wafer based devices such as silicon drift detectors, CCDs and other nanostructural arrays. Furthermore, there was an increase in the visibility of nanotechnology and nanoscience as reflected in several areas of this year’s review. Very large numbers of environmental, industrial and biological/clinical applications were also evident, reflecting the maturity, diversity and increased utilisation of the technique in these areas. This activity may also have been linked to the challenges presented by increasingly stringent environmental and industrial regulations, which continue to be met successfully by XRF technologies and methodologies, though these developments are not always reflected in published references. A notable increase in the use of a range of XRF techniques (conventional, SRXRF and TXRF) in biomonitoring was observed, possibly reflecting new sources of research funding. New uses of SR beamlines for analyses using a range of techniques (micro-SRXRF, EXAFS, XANES, micro-SR XRD, fluorescence and absorption tomography) continued to appear, taking advantage of the availability of very high intensity micrometre-sized beams at modern SR facilities.