Plastic, fill with water, base. Make sure to measure your height allowance if under table
Tip: If you are using this base with a table, umbrella - be sure to measure the clearance under the table. The base itself is really thick, especially if you are used to the flatter, heavy iron ones. Also, fill it after you position it where you want it... cause it's really heavy to move afterward, and there are no wheels. There was a plastic 'leftover' inside. It rattled around. I just left it in there and filled it. Doesn't hurt anything. My 9', 1.5" diameter umbrella fit just fine. I used a silicone seal ring, as well as the screw tighteners. Makes for an impromptu foot rest under the table. I like it so far. It doesn't look as nice as some of the really expensive metal stands, but it's under a table so who cares? lol. It works.