Mold is a microscopic fungus that is part of the natural environment. Mold is all around us, indoors and outdoors. It is a necessary part of our ecosystem. While mold is necessary in the outdoor world, too much mold inside a structure can be dangerous and unsanitary. Mold reproduces through spores that float freely in the air. Once these spores land on a moist surface in the right conditions, mold will begin to grow.