These mattress covers encase the entire mattress (top and bottom) and zipper on.  It was very easy to get on to the mattress IMO.  So far, it’s living up to its promise to be bedbug-free, dustmite-free, and waterproof.  :).  

Note: I cannot speak to comfort (whether it makes you sweat at night or anything like that), as I have a wool mattress topper on top of this mattress cover.