The properties of liquid 4He, and the present understanding of its superuidity, are reviewed. Brief accounts are given of its main physical properties, the 2-fluid model, the Landau excitation model, quantized vortex lines, and critical velocities and their origins. Two experiments in progress on vortex creation and decay are discussed in more detail: an attempt to carry out Zurek's experiment, using the superuid transition to model the GUT symmetry-breaking transition of the early universe; and an exploratory investigation of quantized turbulence in the mechanical vacuum below 50 mK.
Proceedings of Conference, the NATO-Advanced-Study Institute on Patterns of Symmetry Breaking Cracow, Poland, September 15-29, 2002.