A critical decision faced by cells is whether or not to divide.Unicellular organisms evaluate environmental and cellular conditions to make the decision.We investigate the strategy and mechanism of this process in budding yeast, through a combination of mathmatical modeling and single cell expriments.We found that in yeast cells the decision making is dependent on the time integration of a signaling molecule that is sensitive to the environmental conditions.Such a strategy takes into account not only the present status but also the past history of the environment, thus filtering out noise in a fluctuating enviroment.Our findings also shed light on the mechanism of the cell size control at the Start checkpoint .