What are the benefits or negatives of using static classes

Question

Hi,

 

I have been looking at static classes just lately and I use one for passing global variables around an application.

 

I can think of lots of places in my code where I think a static class will work as well as an instantiated class.

 

Could someone please tell me what are the benefits of one type over another and correct instances when one should be used over the other?

 

Thanks,

 

John