The visible universe is a small perturbation on the material universe. Zwicky and Sinclair Smith in the 1930s gave evidence of invisible mass in the Coma and Virgo Clusters of Galaxies. Better optical data has only served to confound their critics and the X-ray data confirms that the gravitational potentials are many times larger than those predicted on the basis of the observed stars. Dynamical analyses of individual galaxies have found that significant extra mass is needed to explain their rotational velocities. On much larger scales, tens of megaparsecs, there is suggestive evidence that there is even more mass per unit luminosity. What is this non-luminous stuff of which the universe is made'? How much of it is there? Need there be only one kind of stuff? There are three basic possi bili ties:- all of it is ordinary (baryonic) matter, all of it is some other kind of (non-baryonic) matter, or some of it is baryonic and some is non-baryonic.

How Many Baryons are There?.- Cometary Masses.- Dark Matter in the Solar System.- Low-Mass Stars and Brown Dwarfs.- White Dwarfs and the Local Mass Density.- Cooling Rates and Numbers of Faint White Dwarfs.- The Galactic Distribution of Neutron Stars.- Wide Binaries and Mass Limits on the Dark Matter.- Dark Matter in the Galactic Disk.- Systematic Properties of Rotation Curves and Dark Matter.- Are There Massive Black Holes in Galactic Nuclei?.- The Formation of Dark Matter in Cooling Flows.- QSO Absorption Lines, Early Evolution of Galactic Halos and the Metagalactic UV Flux.- Baryonic Dark Matter and the Chemical Evolution of Galaxies.- Nuclear Reactions in Inhomogeneous Cosmologies.- Constraints on Baryon-Dominated Cosmological Models from Light Element Abundances and CMB Fluctuations.- Can Halos Consist of Compact Stellar Remnants?.- List of Acronyms.