One-dimensional systems -- Approximate solutions -- Duality of the two-dimensional Ising model -- Combinatorial solutions of the Ising model -- Transfer matrix of the two-dimensional Ising model -- Quantum field theory -- Renormalization group -- Fermionic formulation of the Ising model -- Conformal field theory -- Minimal conformal models -- Conformal field theory of free bosonic and fermionic fields -- Conformal field theories with extended symmetries -- The arena of conformal models -- In the vicinity of the critical points -- Integrable quantum field theories -- S-matrix theory -- Exact S-matrices -- Thermodynamical Bethe Ansatz -- Form factors and correlation functions -- Non-integrable aspects.