Investigation of bacterial survival in natural environments has indicated that some organisms lose culturability on appropriate media under certain conditions and yet still exhibit signs of metabolic activity and thus viability. This reproducible loss of culturability by many bacterial species led to the description of bacterial cells in this state as ‘viable but nonculturable’ (VBNC). It is suggested that the VBNC state is part of the life cycle of non-differentiating bacteria induced by environmental stress. The purpose of this review is to summarize some of the reports which support and refute the hypothesis that the VBNC response is a programmed response. Methods currently used in the determination of viability will be discussed with respect to their advantages and disadvantages. Reports which indicate resuscitation in vitro and in vivo, as well as those that show retention of infectivity and pathogenicity in the case of pathogenic organisms are presented as well.