An investigation has been carried out to examine the usefulness of a computational fluid dynamics (CFD) package to model the collection performance of a range of small sampling cyclones. To further assess the effectiveness and usefulness of CFD. A number of semi- empirical theories were also used to model cyclone sampler performance. The cyclones considered in this study could be used to collect samples of aerosol particles, including airborne microorganisms (bioaerosols). The work described here was carried out as an integral part of a study to select and develop a number of techniques for assessing airborne microorganisms.-lt has been found necessary to characterise bioaerosol samplers with a view that the selection of analysis methods and samplers are interdependent. This publication describes a study carried out to compare the performance of three types of small cyclone samplers using data from CFD, experiments, and three empirical theories. CFD package Fluent 3D has been used satisfactorily to model the performance of three types of small cyclone aerosol samplers. Performance curves were produced with the same shape and 50% cut-off diameter and approximately the same gradients as those obtained by experiment. The predicted pressure drops were also in excellent agreement with the measured data The CFD model is able to predict the salient features of the cyclone how fields in great detail, thus providing a better understanding of the fluid dynamics of these devices. Specifically, results obtained from the computer modelling exercise have demonstrated that CFD is a reliable method of modelling the performance of three types of small sampling cyclones. It has also been shown to be useful for examining the effects of a number of design changes have on their performance. This method of analysis is almost certainly less expensive than experiment, and represents a cost-effective route for design optimisation. Of the three empirical theories considered in this work, only the Earth (1956) theory accurately reflects the performance of the two smaller types of cyclones, provided the design geometries and sampling rates of the cyclone do not deviate too much from the values allied to the ＇＇standard＇＇ version of the sampler. For the larger of the three cyclones considered, only the empirical theory of Iozia and Leith (1989) has been shown to be useful in mirroring experimental performance. The application of empirical performance modelling on the same types of cyclone sampler was not as successful. Performance curves from the Earth (1956) theory matched experimentally derived data for two of the three cyclone designs under consideration. However, for the Stairmand design, the model tended to overestimate the 50% cut-off diameters by between a factor of 2 and 4; and the inclination of the performance curves were reproduced in all cases. This CFD model is a reliable and relatively inexpensive method of examining the effects of design changes on sampler performance of any of these three types of sampling cyclone. It will almost certainly be less expensive than experimental characterisation studies.