  We consider two kinds of periodicities of mutations in cluster algebras. For
any sequence of mutations under which exchange matrices are periodic, we define
the associated T- and Y-systems. When the sequence is `regular', they are
particularly natural generalizations of the known `classic' T- and Y-systems.
Furthermore, for any sequence of mutations under which seeds are periodic, we
formulate the associated dilogarithm identity. We prove the identities when
exchange matrices are skew symmetric.
