Trace and extension theory lay the foundation for solving a plethora of boundary value problems. In developing this theory, one typically needs well-behaved extension operators from a specified domain to the entire Euclidean space. Historically, three extension oper-ators have developed much of the theory in the setting of Lipschitz domains (and rougher domains); those due to A.P. Calder´on, E.M. Stein, and P.W. Jones. In this dissertation, we generalize Stein’s extension operator to weighted Sobolev spaces and Jones’ extension operator to domains with partially vanishing traces. We then develop a rich trace/extension theory as a tool in solving a Poisson boundary value problem with Dirichlet boundary condition where the differential operator in question is of second order in divergence form with bounded coefficients satisfying the Legendre-Hadamard ellipticity condition.