In this paper the complete classification of left invariant metrics of
arbitrary signature on solvable Lie groups is given. By identifying the Lie
algebra with the algebra of left invariant vector fields on the
corresponding Lie group ??, the inner product ??,?? on g = Lie G extends
uniquely to a left invariant metric ?? on the Lie group. Therefore, the
classification problem is reduced to the problem of classification of pairs
(g, ??,??) known as the metric Lie algebras. Although two metric algebras
may be isometric even if the corresponding Lie algebras are non-isomorphic,
this paper will show that in the 4-dimensional solvable case isometric means
isomorphic. Finally, the curvature properties of the obtained metric
algebras are considered and, as a corollary, the classification of flat,
locally symmetric, Ricciflat, Ricci-parallel and Einstein metrics is also
given.