Abstract: Apparatus for controlling at least one variable output parameter in response to a variable predetermined input parameter in a process system, such as a digital thermostat. The apparatus provides adaptive control of the output variable by utilizing a controller means that includes an adaptive controller means, an identifier means and a tuner means. The identifier means defines a model having parameters which represent the operational characteristics of the process system, and the identifier means monitors the operation of the adaptive controller means and selectively changes the parameters of said model to improve the operation of the adaptive controller means. The tuner means receives the model parameters from the identifier means and calculating robust and reliable values of said predetermined gain factors and applying the same to the adaptive controller means for use thereby.