Individual, semistructured interviews were used for the study of 40 primary school students' (ages between 11 and 12 years) conceptions concerning the greenhouse effect. Analysis of the data led to the formation of seven distinct models of thinking regarding this environmental phenomenon. The inferred models are differentiated according to the following criteria: (a) the position and distribution of the greenhouse gases; (b) the existence of connections between the greenhouse effect and the ozone layer, or its depletion; and (c) the types of radiation considered to be involved in the greenhouse effect. However, children's models involve a common core of beliefs, according to which the greenhouse effect is due to solar radiation that heats up the earth and the atmosphere, while getting trapped by certain atmospheric gases, and severely changes the climate, melts the polar ice, and raises the sea level. Furthermore, the alternative conceptions involved in students' models are systematically examined to establish their teaching implications. The main teaching implications discussed here concern: (a) the concept of uniform diffusion of atmospheric gases; (b) the conceptual distinction between ultraviolet and other forms of solar radiation; (c) the conceptual distinction between sunlight and terrestrial radiation; and (d) the conceptual distinction between the roles of the ozone layer and greenhouse gases in the atmosphere. These teaching implications might consititute a valuable research tool for the determination of appropriate educational goals and conditions.