—Northern Fennoscandia is one of the few areas in the world where there are systematic and comparable data on the population density and breeding of the Gyrfalcon during the last 150 years. I have studied population ecology and conservation biology of the species in Finland and nearby areas on the Swedish and Norwegian side of the border since the early 1990s by searching for and monitoring all territories in a defined area. The study area is relatively flat with gently sloping fell-mountains covered by pine and birch forests, the highest tops reaching ca. 1,000 m above the sea level. Ptarmigan form almost 90% of the prey items during the breeding season, about three quarters of them being Willow Ptarmigan (Lagopus lagopus) and one quarter Rock Ptarmigan (L. muta). These two species offer almost the only available food for falcons during winter. The density of Willow Ptarmigan, the main prey species, has varied nine-fold during late winter and early spring from 2000 to 2010, and this fluctuation has had a marked effect on both the percentage of pairs starting to breed and on breeding success of the total population. Territory occupation varied markedly. Of the 25 territories surveyed every year from 2000 to 2010, 12 had breeding pairs every second year or more often, but one-quarter of them were occupied only once or twice during that study period. The median frequency of territory occupancy by a breeding pair was four in 11 years. The percentage of territories occupied by breeding pairs increased from 30–40% to 50–55% from 2002 to 2007 (when Willow Ptarmigan density increased threefold) and reached its maximum two years after the prey population high. In 2009–2010, when the density of Willow Ptarmigan population was exceptionally low, a nine fold decrease from the peak five years earlier, only one-tenth of the Gyrfalcon territories were occupied by breeding pairs. In mid-winter, 90% of the territories were occupied by falcons during the ptarmigan population high, but only one-half of them during the population low. Among 619 records of territory occupancy during the breeding season, all study years combined, pairs began nesting 214 times (34.6%). Of the nesting attempts, 36 (17%) appeared to fail during the egg-stage, but only four (2%) after hatching. Thus, 81% of the pairs that laid eggs were successful in raising at least one young. The number of young per brood varied from one to five, with 2.93 young per successful nesting, on average, and 1.34 young per occupied territory (the respective figures for the main study period 2000–2010, with almost all territories surveyed annually, were 2.98 and 1.40). The mean annual number of big young (probably fledged) per successful