In this paper, we prove a theorem on boundary perturbation of nonautonomous Cauchy problerris arid then apply this result to show the existence and uniqueness of classical solutions of the nonautonomous, Banach space valued functional differential equation0 1999 Academic Press where each A(t) is a linear operator on X . Supposing dense domains D ( A ( t ) ) , one can characterize well-posedness of ( wlCP) by the following definition (see 191). DEFINITION 1.1. The problem ( d C P ) is called wellposed if there exists a unique, strongly continuous evolutionfamily {U(t, s)}, ~ ,~ ~ ~ of bounded, NONAUTONOMOUS FDE'S 159 linear operators on X satisfying 1. U(t, t ) = Id for all 0 I t I T , 2. U(t, s)U(s, r ) = U(t, r ) for all 0 I r I s I t I T , 3. ( t , s) -U(t, s) is strongly continuous, and for every s E [0, TI there exists a dense subspace Y, L X satisfying U(t, 3)x L D ( A ( t ) ) for every 0 I s I t I T such that d -U ( t , S ) X = A ( t ) U ( T , S ) X d t for all x E Y,.