The residential area and population trends and net migration in 331 European cities between 2006 and 2018 were studied.

FREMONT, CA: Population density, which is commonly defined as the ratio between population and occupied area, is a vital indicator of urbanization. The physical growth of cities endangers biodiversity, eliminates farmland, and modifies climate on a variety of scales. Greater density indicates that less area is required to house a given population, freeing up more land for other purposes. Therefore, a study of density variations in space and time to define various patterns and stages of urbanisation has taken on a normative significance. Despite mounting scientific evidence that most urban areas throughout the world are becoming less dense, local governments at various levels push densification as a means of achieving a more sustainable way of life. But the same trends in urban density might mask other forms of urban growth.

De-densification can be used as a sign of suburbanization or sprawl in cities that are expanding in population, but in cities that are contracting in population, it may only be used to signify population loss or to hide a decoupling between population loss and residential area growth. However, even in situations where the population is stable or declining, massive demolitions can nonetheless result in densification, although this is rare. These illustrations imply that disparate urban development paths can lead to similar density trends, each of which has a different effect on the urban environment and the wellbeing of its inhabitants. It is crucial to take into account both of their components' evolution to properly evaluate density patterns as a foundation for policy-making.

In terms of population change, migration, as well as natural change, can influence density trends. The demographic transition, or the change from high to low death and then birth rates as a result of better living conditions, and the mobility transition, or the change from low to high levels of mobility as a result of technological advancements, which favours migration from rural to urban areas, have been identified as the two main drivers of urbanisation. The two transitions effectively describe the initial phases of urban development, albeit their roles and the causal relationships between them are still up for debate. The final stages of the demographic transition, where population expansion is supported by a positive but reducing rate of natural change, have been particularly related to densification.

The suburbanization and inner-city re-urbanization processes observed in some cities in later phases of development, however, are not entirely justified by the two transitions. A "second demographic transition" has been proposed as a potential explanation, which is characterised by fertility rates far below replacement levels and a break between marriage and procreation, leading to fewer households, older populations, and a variety of living arrangements. New migration patterns, which are less tightly tied to the labour market and employment possibilities provided by cities and are more complicated and variable than in the past, are also playing a significant role in driving population expansion. As a result of a number of these factors, location choices are currently more heterogeneous than in the past and are influenced by a wider range of personal preferences than in the past, making it harder to predict how these choices will affect density. This is true, at least for western countries. To support policies intended to promote sustainable urban growth, it is crucial to have a deeper understanding of these tendencies, particularly potential distinct contributions that migration and natural change may make to the spatial development of cities.

This focus on cities in Europe, where a range of regional geographic, climatic, historical, and socioeconomic factors are influenced by a variety of planning traditions and laws, makes Europe a privileged observatory to study the diversity of urban evolution. In addition to this variety, the European Union makes sure that important policy areas, such as spatial development, are coordinated to some extent. The "no net land take" policy, endorsed in 2011 by the European Commission to achieve no net loss of non-urban land by 2050, is particularly pertinent to the discussion of density.