Casting vs As?

Hey all.

I have noticed that in AS3, lets say i have a text field which uses a int variable.

var myNumber:int = 10394;

textField.text = String(myNumber);


textField.text = myNumber as String;

I have to say that i use a mixture of these as i find the need for this situation all the time.

In the past i have noticed that as does not work in some rare cases. I tend to think of these as exactly the same thing but clearly there is a difference.

Can anyone share the specifics?