It works extremely well getting mold and stains off of flat surfaces, porous grout, and tight areas. It will also get stains off of the surface and exposed cracks of caulk , but will not help if you have mold growing inside or behind your caulk.Super easy to use. Just apply and walk away. You're supposed to wipe it off after a few hours, but I typically just leave it. It leaves behind almost no residue.Some people have mentioned that it may damage grout. I have not experienced this. I have used this cleaner on tile, grout, metal, caulk, and linoleum surfaces. So far, I have not noticed any damage or discoloration.Caution:This is a serious cleaner and requires caution and attention. Use sensible precautions (gloves, old clothes, don't touch your face while using, wash hands after use, etc.). After sitting for a while, this stuff has a strong bleach smell. Make sure to use in large or ventilated rooms and open a window for a couple days if possible. If you leave it (instead of cleaning it off), like I do, I would advise closing the door and not spending much time in that room for a couple days. Also, if you have colored grout, you will want to test this in a small area first. It may bleach out your color.