Implement Back Button functionality: Redirect to previous page in ASP.Net

HI

I have 3 page

1-admin.aspx

2-Search.aspx

3-register.aspx

in admin.aspx and Search.aspx I have button that when users click On button they go to register.aspx

I put one image button=IMGBack in register.aspx that I want when users click on it they back to lastpage

I mean if they was in admin.aspx when they click on IMGBack they back to admin.aspx page and if they was in search.aspx they back to search.aspx  page

I want do some thing  like this button (<--) in google toolbar when we click on this button we come back to last page that we visited

HOw I can do it?

Thanks alot