The inventory concept of the NFI links data that were assessed through field surveys, aerial photography interpretation, inquiries and map interpretation, and the informational needs, which were intended to be satisfied with the NFI. Inventory methods are subject to change, since informational needs and technical possibilities of data survey underwent a change through the course of time. In the first NFI, the main objective of the survey was to describe the state of the Swiss forests. This description, which has the nature of a snapshot, was also required in the second NFI. In addition, the changes had to be presented, which took place in the Swiss forest in the ten years between the first and successive inventories. These new problems made further development of the sampling concept of the NFI indispensable.