This article began as an attempt to explain, without assuming any technical physics or advanced mathematics, what the metaphysical (and in particular, the ontological) implications are of the Everett interpretation of quantum theory, often known as the many-worlds interpretation.The difficulty is that physics is not, by and large, gratuitiously technical. It is possible to give a non-technical impression of what Everettian quantum theory is like, but metaphysicians don't just need impressions: they need clear statements of just what a given physical theory is saying.In my view -and this is an underlying theme of the article -this has led to trouble. Physical theories don't, straightforwardly, say anything: the theory, shorn of any interpretation, is a piece of mathematics. In the case of classical point-particle mechanics -that is, in the case of Newton's physics -that piece of mathematics is relatively easy to understand intuitively, at least at a surface level. But in the case of quantum theory, the mathematics is very abstract and its connection to observable facts is both indirect and controversial. There is a great temptation, in trying to communicate quantum physics to philosophers, to tacitly smuggle in controversial interpretative posits as if they were part of the formalism; there is a comparable temptation to present certain, relatively-simply-describable special cases as if they were general; there is, in short, a serious danger of miscommunication, and this is exacerbated by the very different styles of philosophy and modern physics, a difference not entirely recognised by philosophers whose model of physics remains something like Newton's Laws.In this article, I do not pretend to present the mathematics of quantum theory more carefully, in a way that avoids any such communications failure; such a task lies far beyond its scope. My goal, instead, is to give some insight into the features of quantum mechanics, mathematically and interpretationally, that must be understood before any metaphysical discussion of the Everett interpretation can get off the ground. Much of what has been said so far on the subject, I believe, has failed to fully understand those features; as such, it has begun the discussion on the wrong foot. By the end of the article, I will have begun a positive discussion of the Everett interpretation's metaphysical consequences, but I can do no more than begin it without presuming a good 1