The purpose of adaptive fault tolerance (AFT) is to meet the dynamically and widely changing fault tolerance requirement by efficiently and adaptively utilizing a limited and dynamically changing amount of available redundant processing resources. The authors attempt to establish the notion of AFT in a reasonably concrete form, identify major technical issues to be resolved for the practical realization of AFT, and illustrate some feasible approaches to resolving the major issues. After a discussion of the basic concept and major research issues, an important case of AFT management, adaptation to the change of the environment from the soft real-time mode to the hard real-time mode, is examined in some detail. >