The cluster category is a triangulated category introduced for its
combinatorial similarities with cluster algebras. We prove that a cluster
algebra A of finite type can be realized as a Hall algebra, called the
exceptional Hall algebra, of the cluster category. This realization provides a
natural basis for A. We prove new results and formulate conjectures on `good
basis' properties, positivity, denominator theorems and toric degenerations.Comment: 31 pages, typos correcte