The characters of the life cycle and demographic processes of Ixodes ricinus (L., 1758), (Acari: Ixodidae) populations in the North-West of Russia were studied using methods of the long-term observations of ticks in insets in the natural biotopes (2013–2018). It was found that in the North-West of Russia, I. ricinus had a long period of seasonal activity (from April until October) and a long calendar age. Larvae, nymphs, as well as adults of both sexes can remain active for up to 12–15 months when they meet with appropriate host and feed successfully. Up to 9±0.8% of adults can overwinter twice and remain active until the end of June of the third year of their life (23 months). The I. ricinus tick at every developmental stage forms a certain sub-group of population in August–September. Due to the highly prolonged life the larvae, nymphs and females of I. ricinus are able to feed, and the females can lay eggs during their entire active period from April till October. At the same time, larvae and nymphs that feed from April to June molt in the year of feeding, and their development proceeds according to a three-year scenario. The females feeding from April to June can produce offspring during the same season, thus forming a 3-year developmental cycle. By contrast, all individuals (larvae, nymphs and females) feeding from July to October molt to the next stage in August–September of the following year. Females feeding in July–September can lay eggs both in the year of feeding and after winter diapause. This leads to an increase in the duration of the basic 3-year developmental cycle and its complexity. If the ticks molted the next year after feeding only once in the life cycle, the whole life cycle would increase by a year. If they did it in every developmental stage, then the life cycle would increase by three years. Thus, the total duration of development of ticks of one generation may last from 3 to 6 years. Some adult ticks (not more than 9±0.8% in the population) can remain active until the end of June of the seventh season.