A symplectic matrix S∈C2n×2n satisfies S=J−1STJ for . We will consider symplectic equivalence, similarity and congruence transformations and answer the question under which conditions a 3d94da57cb136e1290ced992a34cf0" title="Click to view the MathML source">2n×2n matrix is diagonalizable under one of these transformations. In particular, we will give symplectic analogues of the singular value decomposition and the Takagi factorization.