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The transition from laminar to turbulent flow has been a notorious riddle influid dynamics since the nineteenth century. Hydrodynamic instabilities wereregarded as a cause for the onset of turbulence, but their theoreticalinvestigation led to paradoxical results. I focus on studies about theinstability of laminar boundary-layer flow along a flat plate. The experimentaldiscovery of boundary-layer oscillations (Tollmien-Schlichting waves) in theUSA during World War II vindicated a long-disputed theory developed in Germanyin the 1920s and 1930s. However, the instability which causes theseoscillations marks only the initial phase of the onset of turbulence;higher-order instabilities had to be assumed in order to extend the analysis tothe regions of the transition zone where more complex phenomena (such as"turbulent spots") mark the onset of turbulence. There is still no theory todescribe these processes in a satisfactory manner. The emphasis on thefundamental character of the problem, however, tends to obscure the practicalinterests which motivated the study of boundary-layer instabilities throughoutits long history. The disputed theories played a role in the quest for "laminarairfoils" and other practical applications. Theory and practice are entangledmore intimately than it would appear from the rhetoric about the fundamentalcharacter of the "turbulence problem".

A Kind of Boundary-Layer 'Flutter': The Turbulent History of a Fluid Mechanical Instability