The development of slug flow along two 10 in long inclined pipes (2-90degrees from the horizontal) with internal diameters of 0.024 and 0.054 m was measured by three optical fiber probes. The probes were located in a measurement module at axial distances of 0.020 m between the fiber tips. To measure the evolution of slug flow, the module was placed at different positions along the pipe. Instantaneous elongated bubble velocities and corresponding elongated bubble and liquid slug lengths were determined by processing the optical probe signals. The evolution of the liquid slug and elongated bubble length distributions along the pipes is characterized by a gradual growth of the mean and mode values. The growth rate decreases with decreasing inclination. Mean elongated bubble lengths have a minimum at about 60degrees, while mean liquid slug lengths decrease slowly with decreasing inclination angle. The coalescence rate, defined as the decrease in the ensemble size, becomes almost negligible at x/D > 60, independent of pipe diameter, flow rates and inclination angle. The slug frequency has a maximum at about 60degrees inclination.