The environmental concern of a country's population is an important prerequisite for addressing environmental problems, foremost reducing CO2 emissions and limiting global warming. In this paper, we analyze the development of environmental concern by using the newest wave of the environmental module of the International Social Survey Programme (ISSP) for 29 countries. First, we discuss the measurement of environmental concern and construct a ranking of countries according to the 2020 survey results. Second, we analyze the determinants of environmental concern by employing multilevel models that take individual effects as well as context effects into account. The results show that environmental concern has increased in almost all nations since the last measurement in 2010. The country ranking is headed by European nations such as Switzerland, France and Germany. The USA takes a middle position and China ranks number 20. We observe more variance within countries at the individual level as compared to the differences between countries. At the individual level, environmental concern is closely related to education, post-materialistic values, political attitudes, and individuals' trust in the news media and in science. At the country level, the average environmental concern increases with the wealth of nations.