Signaling between and within cells involves reversible changes in the activity of chemicals, ions, metabolites and proteins. In this Special Focus Issue we have collected new articles investigating the function of biological sensors that detect these changes that occur during signaling. The Editors were keen also to seek the contribution of articles describing the development and use of man-made sensors to measure the in vivo dynamic changes in metabolites and second messengers. Sensors are components that detect, through binding, alterations in the environment, and transduce those alterations to an output. Endogenous cellular sensors that evoke biological responses and man-made sensors used by the experimentalist to measure signaling events should be capable of quantitative measurement of dynamic changes that can occur in milliseconds and could last for several hours. These sensors must be able to respond to the large fold changes in the concentration of hormones, second messengers, ions and metabolites that can occur in the apoplast and the symplast. The endogenous and man-made sensors also need to be capable of responding to and reporting spatially delimited signaling processes that might be restricted to specific organs, tissues, organelles or a subregion of the cytosol. Spatially delimited sensing can be achieved by cell type expression of endogenous and man-made sensors and subcellular targeting of proteins. Endogenous sensors are often restricted to specific regions of the cytosol by tethering to membranes or other signaling components. Man-made sensors are often based on fluorescent proteins to maintain the spatial fidelity of the reported output of the signal.