Techniques to detect Polymorphic and Metamorphic viruses?

Question

What techniques can be applied to detect Polymorphic and Metamorphic viruses?

How difficult is to implement these techniques?

Are these techniques being applied in modern day anti-virus softwares?

Answer 1

I thought most of the virus scanners nowadays use sandbox techniques to check for "bad" behavior. Therefore the polymorphic virusses will also be detected.
of course these detection techniques are also known to virus creators, and can easily be bypassed using a bunch of random, unharmfull, code executions before the actual payload.