Existing research has identified pre-service teachers' (PTs') difficulties with measuring the area of rectangles and explaining their approaches to area measurement. Yet, little is known about elementary PTs' understandings of area measurement. Existing research has focused on children's understanding of area measurement in the context of rectangles and has identified mental processes they use to construct area measurement. The study described in this report contributes to and extends this literature by exploring elementary PTs' understandings of area measurement in the context of rectangles. Clinical interview methodology was used to investigate four PTs' mental processes used in area measurement tasks. Study participants engaged in a 45-minute explanatory interview during which they solved area measurement tasks, including finding areas of rectangles with whole and non-whole number side lengths. Retrospective analysis of interview transcripts was conducted using Battista's framework to identify mental processes used by PTs. While PTs correctly measured the area of rectangles with whole number side lengths, their approaches included identifying side lengths by counting units of area rather than units of length. This approach caused significant difficulties for PTs as they tried to find the area of rectangles with non-whole number side lengths (e.g. length of 0.1 cm). Findings include descriptions of the mental processes used by PTs to find the area of rectangles and are used to describe understanding needed to address measurement of area of rectangles with whole and non-whole number side lengths.