The purpose of this study is to explore students' understanding of loops and nested loops concepts. Sixty-three mechanical engineering students attending an introductory programming course participated in the study. APOS (Action, Process, Object, Schema) is a constructivist theory developed originally for mathematics education. This study is the first attempt to use the APOS framework in the context of programming education. Results showed that APOS theory is a useful framework for examining engineering students' understanding related to loops and nested loops. The results of this study have specific implications for researchers and practitioners when designing programming instruction.