Paintballs, also simply called "paint", are spherical gelatin capsules containing primarily polyethylene glycol, other non-toxic and water-soluble substances, and dye. Early paintballs were made of glass and filled with indelible oil-based paint, since they were made for marking trees and cattle, but modern paintballs should easily wash out of most clothing. The color of the shell does not necessarily indicate the color of the fill.

Most common paintballs and paintball markers are described as .68 caliber, but many factors affect the exact dimensions. Paintballs and barrels vary in size from .67 caliber to .71 caliber. On most paintball markers, the barrel must be slightly larger than the paintball for correct operation.[citation needed] In addition, paintballs are seldom perfectly round and are very sensitive to heat and moisture. A hot or humid day may result in paint swelling or becoming misshapen. Care should be taken to keep paintballs out of the sun and away from moisture. An insulated cooler works well for this on the field.

Generally speaking, paintballs of greater price are subjected to more stringent manufacturing processes, quality checks, and standards, making their size and shape more consistent. This is very important for accuracy. Costlier paintballs also tend to have higher quality fills and thinner shells to improve the frequency of breaking on impact rather than bouncing, and thus raise the percentage of "eliminations."

SWAT teams often use paintballs filled with a pepper substance (known as a pepper ball) as a non-lethal incapacitation method.