  We develop the structure theory of full isometry groups of locally compact
non-positively curved metric spaces. Amongst the discussed themes are de Rham
decompositions, normal subgroup structure and characterising properties of
symmetric spaces and Bruhat--Tits buildings. Applications to discrete groups
and further developments on non-positively curved lattices are exposed in a
companion paper: "Isometry groups of non-positively curved spaces: discrete
subgroups".
