Mitogen-activated protein (MAP) kinases constitute a large family of proteins with many functions. They are represented by a multitude of paralogous isoforms in yeast, vertebrates, and other eukaryotes. A phylogenetically conserved function of MAP kinases is to carry osmotic signals from sensory to target elements of cells. Even though this function of MAP kinases is ubiquitous and characteristic of unicellular and multicellular eukaryotes alike the contingencies between individual MAP kinases, sensor elements, and target elements have been subject to vast modification during evolution. Extensive networking of MAP kinase cascades with other signaling pathways is reflected by the large number of diverse signals that can be carried by a single MAP kinase pathway and flexible activation kinetics. It is emerging that the most important function of MAP kinase networks may not be signal amplification but integration of information about the setpoint of environmental parameters (including osmolality) with other physiological processes to control cell function. Insight into how this cellular integration of information is achieved by MAP kinase networks will shed light on the principles of cell dynamics and adaptation.