The first prerequisite for old-growth forest conservation or restoration is an adequate understanding of the structure and development of old, unmanaged forests. In order to describe the structure and development of old Picea abies-dominated forests we selected a study area from the Paanajärvi wilderness in northwestern Russia. Forest structural characteristics (tree height, diameter, position, age, etc.) were measured in 20 study plots in a sampling grid located north of Paanajärvi National Park. A chronosequence of different forest ages was used to study the late successional development of the forests. The volume of living trees was highest in the youngest age class (110–140 y), whereas that of dead wood increased with forest age. Betula and other deciduous trees occurred as a mixture in all forest age classes, but their proportion of the total volume decreased considerably with forest age. Forest floor microhabitat distribution was most diverse in the oldest age class, where the number of saplings was also highest. Our results show that from 110 to 300 y of age the northern Picea-dominated forests undergo significant structural and compositional changes. In conservation and management of old boreal forests, care should be taken to preserve the full range of this old-growth structural habitat diversity.