The various phases of a project level road pavement rehabilitation investigation and design are identified and discussed in detail. The aim is to optimise the investigation and design of pavement rehabilitation to ensure that enough information is obtained by the design engineer to enable a confident decision to be taken at each of the various phases of the investigation/design. Design methods are caregorised and their limitations and advantages identified and discussed in brief, in order to assist the design engineer to make an informed decision on the use of an appropriate design method(s). Finally, a method is recommended for the life-cycle cost comparison (based on the probability theory) of any applicable rehabilitation options/strategies as designed, in order to make a cost effective recommendation for the rehabilitation of any pavement structure.