The aim of the present study is to shed light on the conceptions that young students have of heat and temperature, concepts that are both important in school science curricula and closely related to daily life. The subjects of the study were students from a rural district in South Korea and they ranged in age from 4 to 11 years. Interviews were conducted with each student on the basis of questions on temperature, thermal insulation, and heat equilibrium. After calculating the frequency and percentage of student responses and analyzing the rationale for their answers, it was found that younger students tended to view temperature as “size” or a “summation of numbers.” This tendency gradually diminished in older students. Most students had alternative conceptions of thermal insulation regardless of age; however, reasoning differed according to age. Younger students displayed a greater tendency to view insulation as a material property, whereas older students showed a greater tendency toward rational heat and temperature conceptions. Most students did not have clear concepts of heat equilibrium regardless of age, but possessed numerous alternative conceptions. © 2007 Wiley Periodicals, Inc. J Res Sci Teach 44: 284–302, 2007