Main problems of modeling the link between aging processes and mechanisms of mortality are addressed. Various applications of Gompertz's law, which allowed to formulate some fruitful hypotheses on the field, are reviewed. Some pitfalls occurring in its applications are also discussed using a model built on purpose to overcome these difficulties. The role played by heterogeneity emerges as the common cause of some relevant failure in using Gompertz's law and the necessary key ingredient of any model aimed to interpret the link between aging and mortality correctly. Though a number of problems are related to inter-individual variability, the search for their solution can lead to an intriguing approach to the study of aging and mortality. Living beings can be considered as complex systems and their age-related changes can be described at the light of complex system theory.