Background Anterior cruciate ligament (ACL) injury is known to have a number of deleterious efects on lower limb muscle function. Alterations in muscle size are one such efect that have implications towards reductions in strength and functioning of the lower limbs. However, a comprehensive analysis of alterations in muscle size has yet to be undertaken. Objective To systematically review the evidence investigating lower limb muscle size in ACL injured limbs. Design Systematic review Data Sources Database searches of Medline, SPORTDiscus, Embase, Cinahl and Web of Science as well as citation tracking and manual reference list searching. Eligibility Criteria for Selecting Studies Individuals with ACL defcient or reconstructed limbs with an assessment of lower limb muscle size and control limb data (contralateral or uninjured control group) Methods Risk of bias assessment was completed on included studies. Data were extracted and where possible meta-analyses performed. Best evidence synthesis was also undertaken. Results 49 articles were included in this review, with 37 articles included in the meta-analyses. 66 separate meta-analyses were performed using various measures of lower limb muscle size. Across all measures, ACL defcient limbs showed lesser quadriceps femoris muscle size (d range=− 0.35 to − 0.40), whereas ACL reconstructed limbs showed lesser muscle size in the quadriceps femoris (d range=− 0.41 to − 0.69), vastus medialis (d=− 0.25), vastus lateralis (d=− 0.31), hamstrings (d=− 0.28), semitendinosus (d range=− 1.02 to − 1.14) and gracilis (d range=− 0.78 to − 0.99) when compared to uninjured limbs. Conclusion This review highlights the efect ACL injury has on lower limb muscle size. Regardless of whether an individual chooses a conservative or surgical approach, the quadriceps of the injured limb appear to have lesser muscle size compared to an uninjured limb. When undertaking reconstructive surgery with a semitendinosus/gracilis tendon graft, the harvested muscle shows lesser muscle size compared to the uninjured limb.