If we want to say what “fundamentality” means, we have to start by approaching what we generally see at the empty place of the predicate “____ is fundamental”. We generally talk about fundamental entities and fundamental theories. At this article, I tried to make a metaphysical approach of what is for something to be fundamental, and I also tried to talk a little bit of fundamental incomplete and complete theories. To do that, I start stating the notion of “entity” and looking at the difference at perceived entities. The difference led us to talk about the entities’ structures and their powers, and about the supervenience between these last two. The supervenience talk made us to see the fight between emergentism and reductionism as the difference between the irreducibility of laws and the reducibility of powers and structures to lower-order domains. Then, we conclude that “fundamentality” is a mereological relation – a relation that a whole structure has to a certain combination of its structural parts or that a power has to a certain combination of its constituent powers – of to be identical and to exist in virtue of them.