A review is given of the observational facts related to the outbursts of dwarf novae and of relevant aspects of the theory of accretion disks. It now appears possible to explain the outbursts of dwarf novae in terms of a nonstationary accretion behavior. Between the outbursts, the material supplied by the secondary component is stored in the relatively cool disk. This eventually leads to a local thermal instability which results in the increase of the viscosity and of the local accretion rate. As the instability propagates across the disk, the global increase in the accretion rate is observed as an outburst. By constructing disk models based on different viscosity approximations and comparing them with observations of dwarf novae it may be possible to obtain some clues concerning the nature of the viscosity. Super-outbursts in the SU UMa objects appear to be a consequence of sudden mass-transfer bursts which are - most likely - due to an instability of the secondary component.