Contents Introduction §0. Agreements, definitions and preliminaries §1. Surfaces of constant negative curvature §2. Measure rigidity of the horocycle flow §3. Geometric generalizations of Ratner's theorem on measure rigidity of the horocycle flow §4. Quotients and joinings of the horocycle flow §5. Rigidity, quotients and joinings of unipotent flows §6. Dynamics of the horocycle flow §7. Classification of ergodic measures for unipotent flows §8. Uniform distribution of unipotent trajectories §9. Various problems of convergence in the space of measures §10. Structure of orbits, minimal sets, and ergodic measures of homogeneous flows §11. Multiple mixing and measure rigidity of homogeneous flows §12. Ergodic measures and orbit closures for actions of arbitrary subgroups §13. Unipotent flows on homogeneous spaces over local fields §14. Applications to number theory §15. Some open problems
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