This book enlarges the class of systems, for which a simultaneously stabilizing controller can be designed, and restricts the class of controllers, from which a solution must exist. The new results here apply to the output feedback stabilization of linear, time invariant, continuous time, single-input, single-output plants. New necessary and sufficient conditions require the existence of an exactly proper controller. For the two plant case, necessary and sufficient conditions are derived only in terms of the plant parameters eliminating the use of the Bezout Identity in determining the existence and the construction of the controller. New sufficient conditions stabilize non-minimum phase plants and relax the high frequency sign condition for minimum phase plants. A new interpolation algorithm is used to create bounded real minimum phase rational polynomials of finite order in constructing simultaneously stabilizing controllers. Generalized sufficient conditions reduce in special cases to results published by several authors. Proofs are constructive so the controllers can be designed, when these new sufficient conditions are satisfied. Examples illustrate the new results.