Protein-modification cycles catalysed by opposing enzymes, such as kinases and phosphatases form the backbone of signalling networks. Whereas historically, kinases have been at the research forefront, a systems-centred approach reveals predominant roles of phosphatases in controlling the network response times and the spatiotemporal profiles of signalling activities. Emerging evidence suggests that phosphatase kinetics are critical for the network function and cell-fate decisions. Protein phosphatases can operate as both immediate and delayed regulators of signal transduction, capable of attenuating or amplifying signalling. This versatility of phosphatase action emphasises the need for systems biology approaches to comprehend cellular signalling networks and predict the cellular outcomes of combinatorial drug interventions.