  In this paper, we consider a class of linear boundary systems with delays in
state, input and boundary output. We prove the well-posedness and derive some
spectral properties of linear system with delayed boundary feedback under some
regularity conditions. Moreover, we show the regularity of linear boundary
systems with delays in state and boundary output. With the above results, the
regularity of linear boundary systems with delays in state, input and boundary
output is verified. As applications, we prove the well-posedness and the
asymptotic behavior of population systems with bounded and unbounded birth
processes "$B_1(t)=\int_0^\infty\int_{-r}^0\beta_1(\sigma,a)u(t-\tau,a)d\sigma
da$" and "$B_2(t)=\int_0^\infty\beta_2(a)u(t-\tau,a)da$", and the
well-posedness of population systems with death caused by harvesting.
