  Lectures presented at the 33rd Karpacz Winter School ``Duality: Strings and
Fields'' briefly introducing dualities in four-dimensional quantum field
theory, and summarizing results found in supersymmetric field theories. The
first lecture describes physical aspects of electric-magnetic (EM), strong-weak
coupling (S), and infrared (IR) dualities. The second lecture focuses on
results and conjectures concerning S-duality in N=2 supersymmetric gauge
theories. The third lecture discusses IR-dualities and their relation to
S-duality in N=1 supersymmetric field theories.
