The future of the world will be urban, with now the largest share of the global population in recorded history living in cities. Urbanization implies a progressive environmental and land-use transformation, from natural ecosystems to artificial materials, shaped by the tension between unregulated organic trends and urban planning. Being the living habitat of most of the current, and likely future generations, growing cities need to remain well functioning, equitable and livable, which includes access to natural areas and the benefits these can provide for urban inhabitants. A key scientific challenge is to understand and quantify these human-nature relationships at scales relevant for cities and urban management. This thesis aims at advancing spatially explicit quantification methods and knowledge regarding accessibility to nature and ecosystem services (i.e. benefits to humans provided by the natural environment) for different urban population groups and in various cities. A main urban study area is the Swedish Stockholm region, while comparative ecosystem service quantifications also extend to and across a large set of European cities. The methods include conceptual developments and spatial modeling for quantification of the targeted urban human-nature relationships. Results show a positive relationship between proximity to green-blue natural areas and income level of urban inhabitants, while dense urban, industrial and commercial areas are less desirable features associated with lower income levels. Income levels also correlate with ethnicity, which thereby also correlates with green-blue area proximity, highlighting an additional spatial segregation perspective for urban regions. A conflict emerging is that people who can afford it choose surroundings with more nature, while further urbanization requires further densification. Care must then be taken not to deplete vital natural areas to the detriment of urban populations, and in particular their less privileged parts. Results also highlight the need to account for the actual spatial connections of humans and their demands for nature’s benefits with the natural areas that can supply these benefits. For example, for the service of local climate regulation (i.e. the ability of natural areas to dampen urban heat island effects and temperature extremes) the thesis investigates conditions in 660 European cities. Results show overall power-law relationships of ecosystem service realization with city population density, but also large variations among cities with similar population densities. Thus, variations in urban forms and land covers, resulting, e.g., from distinct histories and socio-economic evolutions of cities in different countries, lead to measurably better or worse outcomes for provision of the studied ecosystem service. In particular, large divergence is found between cities of eastern and western European countries. Methods developed and results obtained show a practically relevant, comparative quantification approach to cities and their urban ecosystem services as coupled socio-ecological systems, with implications for projection of change trends under urban and economic growth. These can be first steps towards further advancement and improvement needed for spatially explicit quantification and projection of urban ecosystem services and their incorporation in planning, strategy and practice for maintained and enhanced urban well-being.