The form and function of conditional structures in the speech of English speaking children between the ages of 2;6 and 8 years were investigated. Two types of conditionals were distinguished: reality conditionals and unreality conditionals. Data were obtained from audiotapes of the subjects under naturalistic conditions. A five-stage acquisition sequence was described in which the child's increasing ability to deal with abstract notions underlying more complex conditional types lead to mastery of further departures from present reality. From this developmental profile it was seen that the course of conditional development and comprehension generally followed a semantic rather than a grammatical progression. Developmental progress was viewed as the superimposition of successively more abstract layers onto this basic semantic function. (RW)