Sampling has inherent uncertainties when applied tothe sub-micron fraction of natural waters.Processes of aggregation, biological activity andchemical transformation potentially effect changesimmediately after a sample is taken from the waterbody and there is no suitable method of samplestabilization. The size distribution and associatedphysical and chemical parameters of colloids in theaquatic environment can be effectively stable overshort periods of time (about two days under idealconditions, but frequently much shorter timeintervals). To achieve accurate representations of thesize distribution and associated colloidalcharacteristics in situ techniques are required,although adequate approximations may be obtained undersome circumstances if separation is done immediatelyafter sampling. This paper reviews the currentlyavailable strategies for separation and analysis ofcolloids from natural waters (primarily filtration andcentrifugation) and discusses their uses andlimitations, as well as potential uses of promisingtechniques (voltammetry, gels, field-flowfractionation, SPLITT). For small colloids, thetechniques of voltammetry, dialysis, DET and DGT maybe used to obtain in situ information. Forlarger colloids it is more difficult to performmeasurements in situ and a combination of rapidfractionation procedures, including filtration,field-flow fractionation and SPLITT, may still berequired.