Although immunologists know rather a lot about the manif- estation of immunological memory, an understanding of the mechanism of memory at cellular and biochemical levels eludes us. Indeed, as we shall see, it is not even clear which of the several models used to explain the working of memory approximates to the truth. It is in order to report on approaches to this problem and on recent experimental advances in the field of memory cells that this volume has been put together. In the past 4-5 years cell surface molecules that may enable us to define memory Band T cells have been identified. It...