This is an important question because you water filter system is a long term investment and needs to be installed somewhere is can be easily cared for. First, for whole home filters you must have access to the incoming water line. To treat the all of the water in the home you have catch it at its source, before it branches off to other parts of the home. Next the filter needs to be upright; they will not function on their side. That means laying them down in a crawl space is not an option. Finally, they need to easily accessible. Its best when then are installed on a flat, ground-level service where a hand truck can be used to move them. Water filters are heavy, exceeding 200-300 lbs in most cases. They will need service and repairs along the way and being able to easily access you filter will help keep it working properly and save you big money down the road.