This is a status report on additives most frequently used to modify resins, and a review of the mechanisms by which they function. Also are given helpful hints on how to choose those additives that preserve desirable resin properties, add yet others, and facilitate processing. Reviewed additives include - antioxidants, antistatic agents, blowing agents, catalysts, colorants, fillers and reinforcements, flame retardants, impact modifiers, lubricants and slip agents, plasticizers, stabilizers, UV-light stabilizers, and other additives (antiblocking agent, coupling agents, metal deactivators, reodorants, etc).