opening for the top is tight, and when your pack is full and you are putting 3l of water in, it can be tough to get the bladder in. i did figure out how to make it easier, but it took me a while. just lift up (or reverse) the shoulder straps and push back against the pack frame and back panel, which will open the gap up enough for the bladder. you might have to squeeze it in a bit, but it works. once you get the bottom of the bladder in, just shake the pack and let the weight of the water bring it down. there’s a hook 