The molecular signalling cascades which are produced at the cell level are essential for responding to changes in its environment or to alter its homeostasis, e.g. at the cell proliferation, movement, growth or metabolism levels. Understanding the mechanism of signal transduction enables us to explain how the alterations produced in many pathologies are controlled. As such, the changes in signalling can be used to detect the presence of many diseases and the signalling molecules can become therapeutic targets. Furthermore, these signalling pathways are affected in response to therapies used to treat diseases such as cancer, and are involved in creating drug resistance.