MAPK pathways regulate a myriad of cellular functions including proliferation, apoptosis, transformation, and stress signalling. How are these different biological effects coordinated? And how do biochemical pathways make these biological decisions? These are fundamental questions to which we only have incomplete answers. We are using a combination of wet lab experimentation and mathematical modelling in order to investigate how the design and topologies of MAPK pathways contributes to signal integration and biological decision making. I will present work on (i) how competing protein interactions regulate signal processing and biological decision making in the ERK pathway; and (ii) how the ERK pathway crosstalks with stress activated MAPK pathways, such as JNK and p38.