A fundamental responsibility of a scientist is to continue to question how the problems are framed and what presumptions are made.This is particularly true in the field of automatic control,which has been pre-occupied with the study of stability and optimality,premised on the accurate mathematical description of the physical processes.Central to the engineering practice,however,is the problem of intal and extal disturbances,as articulated brilliantly by Prof.H.S.Tsien some 60 years ago.The history of automatic control has been,therefore,the history of two distinct conceptualizations:of stability and optimality on the one hand and of disturbance rejection on the other.This special issue in Control Theory and Technology is a timely snapshot of some current work in the latter.With only seven papers,it is obviously a limited sample of the questions being asked and the progress made.Nonetheless,it points to the necessity and urgency of such a study,which has become increasingly active in recent years.