Calcium ions are perhaps the most versatile ions in the nervous system. In neurons (and other excitable cells) they participate directly in electrical events such as action potentials and slow oscillations, and at synapses they trigger the rapid release of neurotransmitter molecules. Furthermore, calcium ions act as important second messengers for regulating many biochemical processes including those associated with learning and memory and cell death (Ghosh and Greenberg, 1995; Teyler et al., 1994). It is no wonder then that biophysicists and neuroscientists are so interested in measuring changes in the intracellular concentration of calcium ions ([Ca2+]i). Through a number of important technical advances, rapid changes in [Ca2+]i have recently been measured in synaptic nerve terminals (Regehr and Atluri, 1995; Wu and Saggau, 1994), in dendritic spines, and in the large dendritic trees of pyramidal neurons in the hippocampus and neocortex. In the paper by Helmchen et al. on page 1069 of the February, 1996 issue of the Biophysical Journal, the authors have further investigated dendritic [Ca2+]i in pyramidal neurons. In so doing they have addressed a number of thorny issues in the field and have made a potentially important discovery of yet another role for calcium ions in neuronal information processing. The ability to measure fast changes in [Ca2+]i in neurons has come about