A critical review of applications for the period 2000-2003, taken from the Web of Knowledge database, of the techniques high performance liquid chromatography-electrospray ionisation-mass spectrometry (HPLC-ESI-MS) and capillary electrophoresis-electrospray ionisation-mass spectrometry (CE-ESI-MS) to the detection and determination of small molecules of significance in clinical and forensic science is presented. The molecules of mass less than 500 Da are chosen according to selected structural classes in which they give ESI signals primarily as [M+H](+) ions although other ions such as [M-H](-), [M+Na](+) and [M+NH4](+) are also reported. The structural classes are drugs with amine-containing side chains, drugs with N-containing saturated ring structures, 1,4-benzodiazepines, carbohydrates, benzimidazoles, other heterocycles, sulphonylureas, anthracyclines, sulphonamides, penicillins, cephalosporins, tetracyclines, nitrocatechols, steroids, flavonoids, oxazaphosphorines, cannabinols, and miscellaneous molecules. Details are given on the fragmentations, where available, that these ionic species exhibit in-source and in ion-trap, triple quadrupole and time-of-flight mass spectrometers. The review then gives a critical evaluation of these recent HPLC-ESI-MS and CE-ESI-MS analytical methods for the detection and determination of small molecules of clinical and forensic significance. Analytical information on, for example, sample concentration techniques, HPLC and CE separation conditions, recoveries from biological media and limits of detection (LODs) are provided.