Did you ever count bacterial colonies growing in a petri dish? Or track the accumulation of money in your savings account? Or watch as mold spread over an old piece of bread? If so, you have observed population growth in action. As populations grow, the change in the number of individuals in those populations over time can be classified in different ways. Exponential growth and logistic growth are terms applied to specific patterns of population expansion. Both exponential and logistic growth are central to many processes and are the basis for many models.