The three main issues in the foundations of quantum theory are measurement, non-locality, and identical particles (quantum statistics). All of these pertain to the relations between different parts of a composite system. Hence this chapter covers tensor products, reduction of the density matrix (reduced states), and certain aspects of Hugh Everett's relative state formalism. This is then utilized in an extended discussion of measurement as a quantum-mechanical process.
Keywords: density matrix, Hugh Everett, identical particles, measurement, non-locality, quantum statistics, reduced state, reduction, relative state, tensor product