  We establish basic properties of a sheaf of graded algebras canonically
associated to every relative affine scheme $f : X \rightarrow S$ endowed with
an action of the additive group scheme $\mathbb{G}_{ a,S}$ over a base scheme
or algebraic space $S$, which we call the (relative) Rees algebra of the
$\mathbb{G}_{ a,S}$-action. We illustrate these properties on several examples
which played important roles in the development of the algebraic theory of
locally nilpotent derivations and give some applications to the construction of
families of affine threefolds with Ga-actions.
