Material flow in partially filled rotating cylinders (rotary kilns) is encountered in many practical applications of material processing, for example incineration, calcination, grain drying, etc. The flow behaviour in the cross-section is important to other transport mechanisms such as mixing and energy distribution within the bed material. The paper describes an experimental study which was carried out with the objective of understanding and improving our predictive capabilities of the rheological behaviour of granular materials in rotary cylinders. Measurement techniques similar to that used in chute flows have been employed to measure flow characteristics, e.g. particle velocities, granular temperature, and solid concentration (in the shear layer developed between the free surface and the bulk of the bed) for different materials having a wide range of coefficients of restitution. The results of the experiments provide the necessary assumptions, constraints, and data for granular flows in partially filled rotating cylinders.