  In representation theory of algebras the notion of `mutation' often plays
important roles, and two cases are well known, i.e. `cluster tilting mutation'
and `exceptional mutation'. In this paper we focus on `tilting mutation', which
has a disadvantage that it is often impossible, i.e. some of summands of a
tilting object can not be replaced to get a new tilting object. The aim of this
paper is to take away this disadvantage by introducing `silting mutation' for
silting objects as a generalization of `tilting mutation'. We shall develope a
basic theory of silting mutation. In particular, we introduce a partial order
on the set of silting objects and establish the relationship with `silting
mutation' by generalizing the theory of Riedtmann-Schofield and Happel-Unger.
We show that iterated silting mutation act transitively on the set of silting
objects for local, hereditary or canonical algebras. Finally we give a
bijection between silting subcategories and certain t-structures.
