Cremona groups are the groups of birational transformations of a projective space. The structure of these groups depends on the dimension of the projective space, and on the field over which the transformations are defined. In this talk I consider the Cremona group of the plane over a perfect field and proof that they are generated by involutions. I will explain how to decompose such birational maps into Sarkisov links and how this gives a generating set of the plane Cremona group. Afterwards, I will decompose them into involutions, among them are Geiser and Bertini involutions as well as reflections in an orthogonal group associated to a quadratic form. This is joint work with Stéphane Lamy.