Since the last quarter of the 19th century, the status of the second law of thermodynamics has been fiercely debated. Boltzmann saw the 2nd law as a consequence of the complexity of molecular motions. Loschmidt argued that irreversibility cannot be derived from a fundamentally time-reversible dynamic principle.   In this talk I will describe a special class of dynamical systems for which the "Loschmidt Echo" can be observed and what such observations tell us about irreversibility and the arrow of time