Abstract
We introduce the key ideas and mathematical tools central to modern discussions of collapse. We assume knowledge of basic quantum mechanics -- the Schr\"{o}dinger evolution of states in Hilbert space, the Born probability rule, the Dirac formalism, and so on -- while explicating such ideas as projectors, density operators, Bell inequalities, entanglement and the Lindblad equation. In other words this work is a \emph{primer} in the simplest sense of that word: without taking a stand on the major questions, it is intended to make an enormous and growing body of research more accessible.