Knowledge of the global structure of the Galactic magnetic field (GMF) is very hard to obtain, and until recently models have been seriously incomplete. I will describe work done in collaboration with R. Jansson to determine the GMF using Rotation Measures of 40,000 extragalactic sources and the WMAP synchrotron emission maps. To get a good description of that data, we developed a 22-parameter model of the coherent and striated components of the GMF and a 13-parameter model of the random field. The data required us to include features in the model that had been previously overlooked. Implications and open questions will be discussed.
