Brake Pad shims - greatspirit
On both my MX-5 & my Punto there are thin metal shims between the pad and the piston. These are usually corroded and new ones are not included with new pads. What is their purpose ans can they be dispensed with?
Brake Pad shims - Dynamic Dave

They're called "anti-squeal shims"

The name suggests it does what it says on the tin.

No idea if they an be dispensed with or not.

Brake Pad shims - solara
Best keep them in place, even if they are corroded (could always try cleaning them).
It is basically a piece of softer metal (sometimes copper) which prevents them from squealing (as the name suggests). Haynes manual usually advises to put a small bit of cooper grease on the mating surface of the shim.
Brake Pad shims - defender
they also help to stop heat travelling from the pads to the piston .some pads have a thin rubber bit between the two ,you could make them with very thin tin.
Brake Pad shims - Snakey
Nearly all the cars I've had recently had have these fitted as standard when 'new', and when I've put replacement pads on without them I've coated the backs with copper grease as described and had no problems or noises.

Apart from when I bought some pads for my Omega which apparently are also designed to fit a Nissan Almera. I guess they must fit the Almera better as they rattled around inside the caliper frame!
Brake Pad shims - Mikey Jay
The shims will probably only fit the type of pads fitted as original equipment on your car. They may not fit generic type pads bought at your local motor factors as these pads are designed to fit as many different vehicles as possible. Mike.
Brake Pad shims - Civic8
As DD said but I have never had a problem leaving them off.But make sure back of pad to piston/inner caliper has copperslip on not on contact areas to disc.the shims can be a pain to stop squealing if they do start.