  The onset of turbulence in laminar flow of viscous fluids is shown to be a
consequence of the limited capacity of the fluid to withstand shear stress.
This fact is exploited to predict the flow velocity at which laminar flow
becomes turbulent and to calculate, on a theoretical basis, the corresponding
critical value of the Reynolds number. A constitutive property essential to the
present analysis is the ultimate shear stress of the fluid. The paper shows how
this stress can be determined experimentally from a test in plane Couette flow.
For water at 20 {\deg}C, the value of the ultimate shear stress is calculated
from the experiments reported in the literature. This value is then is employed
to predict the Reynolds number corresponding to the onset of turbulence in
Taylor-Couette flow and in pipe flow of circular cross section. The results are
realistic and their significance is assessed critically. The procedure can be
applied to predict the onset of turbulence in any non-turbulent flow, provided
that the velocity field of the flow is known.
