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Surface photometry shows that most spiral and S0 galaxies have two main components: a spheroidal component, and an exponential disk component with radial surface-brightness distribution 1(R) = Ioe 5. The exponential disk is the subject of this paper. First, for the exponential disk in centrifugal equilibrium with surface density (R) = , we derive the circular-velocity field and the mass- angular momentum distribution (h); (h) is the total mass with angular momentum per unit mass less than h. (h) for the exponential disk is almost identical with (k) for a family of rigidly rotating spheres of uniform density. We then collect photometric data for the disks of thirty-six spiral and S0 galaxies, and find the following: (i) Twenty-eight of the thirty-six galaxies have approximately the same intensity scale 1o (21.65 B-mag per square second of arc), with a standard deviation of only 0.30 mag per square second of arc, despite a range of nearly 5 mag in absolute magnitude. This constancy of 1o produces the correlation between apparent magnitude and angular diameter found by Hubble. (ii) S0-Sbc systems have any value of the disk length scale between 1 and 5 kpc, while later-type systems have predominantly low values of a (<2 kpc). (iii) The relative brightness and size of the spheroidal and disk components are only weakly with morphological type. If conclusion (i) implies that s is approximately constant, then the disk's total mass and angular momentum satisfy 7/4 If (h) is invariant as a protogalaxy collapses to form a galaxy, then all protogalaxies destined to be S0 or spiral galaxies have a similar (h) (in dimensionless variables), at least for the range of h corresponding to the disk. If (k) is not invariant, then there exists a very efficient mechanism which establishes the characteristic ) (h) for these systems as they form. The exponential nature of the disk is not defined by i) alone; its cause remains uncertain.