The article presents the concept of Ecological Footprint (EF), which is a quantitative indicator of human impact on the environment. The idea of EF has originated from the concept of carrying capacity. The Ecological Footprint measures how much of the land and water area a human population requires to produce the resource it consumes and to absorb its wastes, using the prevailing technology. The methodology was developed by Rees and Wackernagel (1996). The Ecological Footprint Assessment is a common supporting tool in planning and development of cities, subnational geographical regions and states. EF is important in ecological education at the primary and higher educational level, also including academic grade. At the beginning of the 21st century, requirements of the population in some countries (e.g. U.S., United Arab Emirates, Kuwait, Denmark, Australia, Canada) already exceed the planetary limits and ecological assets are becoming more critical. Implementation of the EF concept demands precise definition of many terms taken from ecology, geography, technology, or economy. The most important terms are explained in the glossary. More than half the global population (on average about 51%) live in cities (in Poland about62%). Their inhabitants have a substantial impact on the environment. The EF value for inhabitants of the capital city of Poland – Warsaw – in 2005 was 6.5 gha per capita, for the inhabitants of Cracow – 7.67 gha per capita. The average EF worldwide value in 2005 was approximately 2,1 gha per capita, and in 2007 1.8 gha per capita. The inhabitants of Warsaw and Cracow, through consumption of goods and services, exert significant pressure on the environment and aggravate the ecological deficit of the Earth.