Despite recent developments in the understanding of boundary layer receptivity and non-linear stability, linear stability methods remain the state-of-the-art in industry for aerodynamic design and analysis. A conceptual model is presented to explain why the eN approach is used and the circumstances under which it might be expected to work. The paper reviews the latest results and conclusions from a series of recent collaborative projects, supported by the European Commission, which have contributed significantly to the confidence and ease with which linear stability methods can now be used for design. Recent experimental work has allowed local, linear stability N-factor correlations to be derived, for the first time in Europe, for HLF systems. A range of N-factor integration strategies have been evaluated during the analysis of these experiments. The use of non-local theory has demonstrated a significant effect on crossflow N-factors which warrants further, systematic correlation of these N-factors against experiment. The authors feel that the use of advanced non-linear transition prediction techniques can be used to provide guidance in the avoidance of pathological situations in the design of commercial HLF systems, but that linear stability theory is today's best tool for design purposes. Database methods derived from linear theory can considerably accelerate the design process provided that they are validated appropriately against stability computations.