A method is given for obtaining the reliability indexes (RI) of control which is based on the control target criterion (CTC). CTC reflects the probability of the object belonging to the working state (to the state adequate to the control task), and is analogous to the fuzzy logic member ship function. RI takes into account the probability density of the controlled parameter (CP), the choice of which provides the agreement of CTC with the estimate of CTC determined by experts.. In RI, methodological and instrumental components are distinguished, which together form the total RI (probabilities of false diagnosis and under-diagnosis). The methodological component gives the attainable limit of RI for precise measurement of CP. Using the given CTC and RI, the solution of  the inverse problem allows one to estimate the acceptable control levels and measurement precision. Examples illustrate the CTC for medico-technical control of heart and breathing rates of operators in imitator conditions, and CTC  for radiation control using the scheme "radiation dose - biological damage".