In this paper the authors present linear matrix inequality (LMI) conditions for output feedback control problems. The results based on sufficient conditions because they are dependent on the particular state-space representation used for describing the system. Nevertheless, the conditions are not sensitive to a certain class of state-space transformations, and if the control problem is feasible then there exists some state-space transformation leading the conditions to be necessary and sufficient for the problem. The authors approach can be used for designing decentralized controllers and is easily extended to H-2, H-infinity and mixed H-2/H infinity problems via standard LMI techniques. The continuous- and discrete-time cases are considered and numerical examples are given to illustrate the results.