Firstly be aware that, when brand new, these can shed off synthetic material into your wort, so wash them first to prevent this. Secondly, as others have mentioned, tiny hop and/or grain material will go through the mesh holes. Take that for what it's worth. Thirdly, and most importantly, be VERY CAREFUL not to allow these bags to touch the hot bottom of the wort pot while steeping. If these contact the hot bottom of the pot, they will melt/burn. The burn results in a nasty gunk sticking to the bottom which will contaminate the wort. The bag will then burst open, spilling the grains or hops or whatever into the wort. CAREFUL!!!!However, they still work really well with all this considered! Can re-use a couple times, too (only need one for a whole batch).