Basic classes of matrices or linear transformations in finite dimensional quaternionic vector spaces with a regular indefinite inner product are introduced and studied. The classes include plus matrices, selfadjoint, skew-adjoint, and unitary matrices. In particular, results are proved concerning extensions of invariant semidefinite subspaces. Canonical form for unitary matrices is developed and subsequently applied to stability of periodic Hamiltonian systems. © 2005 Elsevier Inc. All rights reserved.