This paper presents a review and analysis of the results of recent research that has been carried out on horizontal pneumatic conveying of materials in the dilute phase. An introduction to dilute phase pneumatic conveying is given. Many in-process applications require a detailed knowledge of the cross-sectional particle concentration distribution and an insight into the research carried out in that field is reviewed. Tomography and computational fluid dynamics (CFD) modelling have been identified as the available tools for achieving the aims of such a study and an overview of each is presented. Recent research has concentrated on identifying the optimum operating conditions to increase the energy efficiency of the systems and reducing pipe wear. A developing research area has been the modelling of particle behaviour within pipe systems, especially after bends. Gravitational settling in horizontal pipes, inertial behaviour in pipe bends and. branches, turbulent dispersion, turbophoresis and transverse lift forces induced by particle rotation were reported to be the effects governing particle motion that ought to be accounted for by a dilute gas-particle flow model. Particle size, solids loading ratio, pipe material and bend characteristics were identified to be the controllable parameters that determine cross-sectional particle concentration distribution. The effect of secondary turbulent flows and lift forces are enhanced by wall roughness and particle-wall interactions. A new technique of swirling flow pneumatic conveying (SFPC) has been recently applied. However. numerical modelling of such flows is still at an early stage because the mutual coupling between the two phases is not well understood. The physical understanding and modelling of the cross sectional distribution of particles in dilute SFPC has been identified as a challenging area for future research. (C) 2003 Published by Elsevier Science B.V.