  Low-dimensional quantum spin systems are interacting many body systems for
which several rigorous results are known. Powerful techniques like the Bethe
Ansatz provide exact knowledge of the ground state energy and low-lying
excitation spectrum. A large number of compounds exists which can effectively
be described as low-dimensional spin systems. Thus many of the rigorous results
are also of experimental relevance. In this Lecture Note, an introduction is
given to quantum spin systems and the rigorous results known for such systems.
Some general theorems and exactly-solvable models are discussed. An elementary
introduction to the Bethe Ansatz technique is given.
