Check out the replies in this thread below. Will give some more points to your query..

http://www.8052.com/forum/read.phtml?id=63423

And it would be well if you can ask a more focussed question :

The testing requirements of a remote controlled toy car is different from,

the requirements of a Engine control unit in an automobile which is different from,

the requirements of a Mars lander communication module which is different from,

the requirements of a pace maker electronics.

Raghu