A practical evaluation of the possibilities and limitations of ultra performance liquid chromatography (UPLC) is presented. Acquity BEH columns packed with 1.7 microm particles are evaluated by means of van Deemter and Knox plots. The columns are characterised by high optimal velocities (3.7 mm/s) and low plate heights (4.4 microm). Minimum plate heights of 2d(p) were, however, not reached and reasons are presented and discussed. Furthermore, the use of 1.7 microm particles at 1000 bar is compared, from a theoretical viewpoint, to conventional LC (3.5 and 5 microm particles at 400 bar) in terms of analysis speed and practical maximum efficiency. Experimental data are used to construct kinetic- or "Poppe-plots", which facilitate investigation of the effect of pressure and particle size on speed and efficiency. It is found that UPLC conditions hold advantages in terms of speed of analysis, for required theoretical plate counts up to approximately 80,000.