A bibliographical review on the convective heat transfer through microchannels is presented. The available experimental works quoted in the open literature are critically analysed in order to highlight the main results obtained on the friction factor, on the laminar-to-turbulent transition and on the Nusselt number in channels having a hydraulic diameter less than 1 mm. A comparison of the experimental results quoted in the open literature is made. In many cases the experimental data of the friction factor and of the Nusselt number in microchannels disagree with the conventional theory but they also appear to be inconsistent with one another. Various reasons have been proposed to account for these differences. Rarefaction and compressibility effects, viscous dissipation effects, electro-osmotic effects (EDL), property variation effects, channel surface conditions (relative roughness) and experimental uncertainties have been invoked to explain the anomalous behaviour of the transport mechanisms through microchannels. By comparing the available experimental data on single-phase convective heat transfer through microchannels, it is evident that further systematic studies are required to generate a sufficient body of knowledge of the transport mechanism responsible for the variation of the flow structure and heat transfer in microchannels.