Giant unilamellar vesicles, prepared by hydrating a synthetic lipid, are visible under phase-contrast microscopy. Additives injected into or onto the vesicles induce various morphological changes that have been recorded photographically. For example, KI/I-2 creates a large, slowly-healing hole in the vesicle surface to form a solvent-filled "nanocup". Sodium cholate (a bile salt) also injures the vesicle surface, but the defect is smaller and heals rapidly. Sodium acetate induces vesicle fusion, a process explainable by the Svetina-Zeks mechanism. Poly(vinyl alcohol) causes filament-connected vesicles (but not isolated vesicles) to fuse. This observation leads to the speculation that many fusion experiments with submicroscopic vesicles might also reflect, unknowingly, the presence of intervesicular filaments. Severe osmotic stress, as provided by 0.1 M NaBr, forces the vesicles to undulate vigorously. Finally, injection of a fluorescent dye into the vesicles allows, via fluorescent microscopy, the detection of outward diffusion by the dye. Giant vesicles provide a particularly valuable membrane model because, unlike submicroscopic vesicles on which the bulk of bilayer research has thus far been focused, the results are not affected by an unnatural membrane curvature. Moreover, morphological changes can be monitored as a function of membrane composition and experimental conditions without relying on indirect spectroscopic methods.