Background. Rehabilitative ultrasound imaging (RUSI) measures of abdominal wall muscles are used to indirectly measure muscle activity. These measures are used to identify suitable patients and to monitor progress of motor control exercise treatment of people with low back pain. Purpose. The purpose of this study was to systematically review reproducibility studies of RUSI for measuring thickness of abdominal wall muscles. Data Sources. Eligible studies were identified via searchcs of MEDLINE, EMBASE, and CINAHL. The authors also searched personal files and tracked references of the retrieved studies via the Web of Science Index. Study Selection. Studies involving any type of reliability and or agreement of any type of ultrasound meisurcrnents (B or M mode) for any of the abdominal wall muscles were selected. Data Extraction. Two independent reviewers extracted data and assessed methodological quality. Data Synthesis. Due to heterogeneity of the studies' designs, pooling the data for a meta-analysis was not possible. Twenty-one studies were included, and these studies were typically of low quality and studied subjects who were healthy rather than people seeking care for low back pain. The studies reported good to excellent reliability for single measures of thickness and poor to good reliability for measures of thickness change (reflecting the muscle activity). Interestingly, no studies checked reliability of measures of the difference in thickness changes over time (representing improvement or deterioration in muscle activity). Conclusions. The current evidence of the reproducibility of RUSI for measuring abdomiial muscle activity is based mainly on studies with suboptimal designs and the study of people who were healthy. The critical question of whether RUST provides reliable measures of improvement in abdominal muscle activity remains to be evaluated.