  When considering fractional diffusion equation as model equation in analyzing
anomalous diffusion processes, some important parameters in the model related
to orders of the fractional derivatives, are often unknown and difficult to be
directly measured, which requires one to discuss inverse problems of
identifying these physical quantities from some indirectly observed information
of solutions. Inverse problems in determining these unknown parameters of the
model are not only theoretically interesting, but also necessary for finding
solutions to initial-boundary value problems and studying properties of
solutions. This chapter surveys works on such inverse problems for fractional
diffusion equations.
