  The purpose of this paper is to introduce a semigroup approach to linear
integro-differential systems with delays in state, control and observation
parts. On the one hand, we use product spaces to reformulate state-delay
integro-differential equations to a standard Cauchy problem and then use a
perturbation technique (feedback) to prove the well-posendess of the problem, a
new variation of constants formula for the solution as well as some spectral
properties. On the other hand, we use the obtained results to prove that
integro-differential systems with delays in state, control and observation
parts form a subclass of distributed infinite-dimensional regular linear
systems in the Salamon-Weiss sense.
