Following a lucid investigation about what galls are, the author delves into a discussion of the host plants and causal agents. Among the host plants, galls or cecidia are found more commonly on dicots than on monocots, gymnosperms, or cryptogams. Galls are found on all types of plants and all types of plant parts with more variety found on aerial than subterranean parts. A discussion of gall structure introduces the discourse about galls on specific plant parts: roots, shoots, buds, leaves, flowers, and fruits.