This paper is concerned with the design of reconfigurable control laws that also effectively facilitate the acquisition of information used for diagnostic purposes in fault tolerant control systems, A control performance index and a diagnostic performance index are chosen to indicate the overall performance, A simple relationship between the two indexes is established that elucidates the control/diagnostic tradeoff, A control design method is developed that first finds a set of controllers achieving a certain control performance level and then uses the remaining degrees of freedom in the controller set to achieve an optimized diagnostic performance level.