This graduate-level text reviews the field of critical phenomena, including the use of neutron scattering techniques as an aid in their study. The book introduces the principles of magnetic systems and their critical dynamics, outlining the experimental and theoretical methods that have been used to understand the scattering effect. Measurements are examined for the dynamics and statics of one-, two-, and three-dimensional systems. Multicritical effects, critical phase transitions in magnetic metals, dilution, percolation, and random-field effects are also discussed in the light of neutron scattering measurements.