Harmonious, integrated functioning of the whole plant system requires that its various cells, tissues and organs should be able to communicate with each other, transferring a range of information on environmental conditions, physiological and microbial stresses etc. In this volume of Advances in Botanical Research incorporating Advances in Plant Pathology three articles are concerned with different aspects of plant signalling. McDonald and Davis consider how shoot systems respond to drying and N-deficient soil, in terms of their stomatal behaviour and growth, via the transmission of root-derived chemical signals. Malone considers the major hypotheses that have been proposed with particular attention being given to hydraulic pressure signals and the hydraulic dispersal of chemical signals. At a different, intracellular level of communication, a wide variety of second messengers couple extracellular stimuli to a characteristic physiological response. Webb et al. Consider progress made in establishing similar roles for calcium in plant signalling in the context of the mammalian paradigms. The effects of UV-B radiation on plants have been extensively investigated in recent years. Jordan considers progress in understanding the chain of events from perception of UV-B to signal transduction and consequent changes in gene expression and regulation. Smith and Smith assess the various hypotheses erected over the years to explain structure and function of the host-parasite interface formed by vesticular-arbuscular (VA) mycorrhizas, an important and widespread mutualistic symbioses of a wide range of higher and some lower plants.