They should be taken out each time that you take a shower or bath to prevent the buildup of mold and mildew under the mat. Even though some of the models have large holes in them for great water drainage, some of the water will remain under the mat. This is a perfect breeding area for bacteria and harmful substances. It only takes a few seconds to pick up the mat and hang it up. You will be glad you did, plus it sets a good example for kids.