Lee, LiTits, A.L.The "median phase" and "phase spread" of a matrix are defined and properties are derived. The question of robust stability under uncertainty with phase information is addressed and a corresponding necessary and sufficient condition is given. This condition involves a "phase sensitive singular value". A computable upper bound to this quantity is obtained. The case when the uncertainty is block-structured is also considered.en-USlinear systemsrobust controlstabilitysystem theoryIntelligent ServomechanismsTechnical Report