Abstmcf-This paper discusses image segmentation techniqug from the standpoint of the assumptions that an image should satisfy m order for a particular technique to be applicable to it. These assumptions, which are often not stated explicitly, a n be teguded as @erhrps i n f m a l ) "models" for citsses of images. The papex emphrsizg two basic durses of models: m Z models that desaibe the pixel population in an image OI region, and models that descn'be the decomposition of an image into regions.