A landmark study published over ten years ago established a link between mutations of the LKB1 gene and the Peutz-Jeghers cancer syndrome. Since then, much effort has been devoted to understanding the functional roles of the protein encoded by the LKB1 gene. LKB1 is a Ser/Thr kinase that is activated by binding to the pseudokinase STRAD and the scaffolding protein MO25. LKB1 activates AMPK and twelve other members of the AMPK family of kinases, thus controlling cellular energy and cell polarity. Much attention has been focused towards protein kinases activated by LKB1, although the mechanism by which STRAD and MO25 activate LKB1 remains unknown. The work described in this thesis was conducted with the aim of understanding the interactions of LKB1 with STRAD and MO25, and the mechanisms by which the latter two activate LKB1. I have tried to accomplish these aims using a structural biology approach. In chapter III of this thesis I show the structure of the regulatory STRAD /MO25 complex, and in chapter IV the structure of the heterotrimeric LKB1/STRAD /MO25 complex. The structure of STRAD reveals for the first time the determinant features of kinase inactivity, and provides an example of how pseudokinases can exert their functions through their conformation rather than phosphorylation. STRAD adopts a closed conformation typical of active protein kinases, and binds LKB1 as a pseudosubstrate. STRAD binding promotes the active conformation of LKB1, which is further stabilised by MO25 interacting with the LKB1 activation loop. This represents a previously undescribed mechanism of kinase activation that may be relevant to understanding the evolution of other pseudokinases. Finally the structure of the LKB1 heterotrimer reveals how mutations found in Peutz-Jeghers syndrome and other cancers impair LKB1 function.