  The numerical hydrodynamic modelling of beat Cepheid behavior has been a
longstanding quest in which purely radiative models have failed miserably. We
find that beat pulsations occur naturally when turbulent convection is
accounted for in our hydrodynamics codes.
  The development of a relaxation code and of a Floquet stability analysis
greatly facilitates the search for and analysis of beat Cepheid models.
  The conditions for the occurrence of beat behavior can be understood easily
and at a fundamental level with the help of amplitude equations. Here a
discriminant D arises whose sign decides whether single mode or double mode
pulsations can occur in a model, and this D depends only on the values of the
nonlinear coupling coefficients between the fundamental and the first overtone
modes. For radiative models D is always found to be negative, but with
sufficiently strong turbulent convection its sign reverses.
