A review of gas flow in microchannels of different cross-sections is presented in this article. The phenomenon of slip, slip models and measurement of slip coefficient are first discussed. The planar microchannels are dealt with in great detail; the different analytical, experimental and simulation approaches and solutions available in the literature are reviewed. A review of literature on square, rectangular and circular microchannels have revealed that these microchannels have also been extensively studied. Information on other shapes of microchannels such as triangular, trapezoidal and elliptical is also presented. It is recognized that surface roughness may play an important role; therefore the effect of surface roughness on flow characteristics and overall pressure drop is given due attention. It is further recognized that frequently the microchannels may not be straight; therefore, microchannels with bends or involving sudden expansion/contraction is considered. Empirical correlations and equations are included for ready reference.