This chapter describes the importance of population monitoring, a complex task with many interrelated parts. The theoretical foundation for monitoring lies in the random sample and the field of statistics. Effective survey design provides the building blocks for monitoring all relevant quantities in a general and reusable fashion. Population models help to formalize hypotheses, design monitoring programs, find efficient uses of resources, and make proactive assessments of management actions. By combining the guidance received from population models with strict sampling procedures, the state of the population can efficiently be established, which facilitates adoption of the scientific method and rigorous decision making.