Cells respond to a plethora of signals using a limited set of intracellular signal transduction components. Surprisingly, pathways that transduce distinct signals can share protein components, yet avoid erroneous cross-talk. A highly tractable model system in which to study this paradox is the yeast Saccharomyces cerevisiae, which harbors three mitogen-activated protein kinase (MAPK) signal transduction cascades that share multiple signaling components. In this review we first describe potential mechanisms by which specificity could be achieved by signaling pathways that share components. Second, we summarize key features and components of the yeast MAPK pathways that control the mating pheromone response, filamentous growth, and the response to high osmolarity. Finally, we review biochemical analyses in yeast of mutations that cause cross-talk between these three MAPK pathways and their implications for the mechanistic bases for signaling specificity. Although much remains to be learned, current data indicate that scaffolding and cross pathway inhibition play key roles in the maintenance of fidelity.