The structure and evolution of solar supergranulation is studied using horizontal velocity fields, deduced from applying local correlation tracking (LCT) techniques to full-disk, line-of-sight Doppler velocity data observed by the Michelson Doppler Imager on board the Solar and Heliospheric Observatory spacecraft. Two 45° square regions of photospheric plasma, one of the quiet Sun and one with increased magnetic activity, are tracked for as long as they remain visible on the disk of the Sun (about 6 days), enabling a determination of the complete life histories of over 3000 supergranules in each region. With this method, the horizontal outflows associated with the pattern of supergranulation are revealed with clarity, even for locations near disk center where little of the horizontal velocity field is projected into the line of sight. The LCT flow mappings are of sufficient temporal extent that they can be used to study the complex evolution of a broad spectrum of supergranules, revealing that merging and fragmentation events figure prominently in the life histories of more than half of the supergranules in each data set. Such dynamics lead to many short-lived supergranules (about 75% of the total population) having lifetimes of less than 24 hr, coexisting among numerous long-lived supergranules, many of which exist for several days. Average supergranular lifetimes lie in the 16-23 hr range, although about 7% of all are recognizable for time periods of 48 hr or more. The average supergranular cell diameter lies in the 12-20 Mm range, with smaller cells more prevalent in areas of greater magnetism. There exists a tendency for larger cells to preferentially have longer lifetimes when embedded in a region of increased magnetic flux.