  The present monograph explores the correspondence between quantum and
classical mechanics in the particular context of spin systems, that is,
SU(2)-symmetric mechanical systems. Here, a detailed presentation of quantum
spin-j systems, with emphasis on the SO(3)-invariant decomposition of their
operator algebras, is followed by an introduction to the Poisson algebra of the
classical spin system and a similarly detailed presentation of its
SO(3)-invariant decomposition. Subsequently, this monograph proceeds with a
detailed and systematic study of general quantum-classical symbol
correspondences for spin-j systems and their induced twisted products of
functions on the 2-sphere. This original systematic presentation culminates
with the study of twisted products in the asymptotic limit of high spin
numbers. In the context of spin systems, it shows how classical mechanics may
or may not emerge as an asymptotic limit of quantum mechanics.
