Background. In order to make inferences about strength related to development or treatment interventions, it is important to use measurement instruments that have sound clinimetric properties. Purpose. The objective of this review is to systematically evaluate the level of evidence of the clinimetric properties of instruments for measuring upper extremity muscle strength at the "body functions & structures" level of the International Classification of Functioning, Disability and Health for Children and Youth (ICF-CY) for children with cerebral palsy (CP). Data Sources. A systematic search of the PubMed, EMBASE, OTseeker, CINAHL, PEDro, and MEDLINE databases up to November 2012 was performed. Study Selection. Two independent raters identified and examined studies that reported the use of upper extremity strength measurement instruments and methods for children and adolescents with CP aged 0 to 18 years. Data Extraction. The COSMIN (COnsensus-based Standards for the selection of health status Measurement INstruments) checklist with 4-point rating scale was used by 2 independent raters to evaluate the methodological quality of the included studies. Best evidence synthesis was performed using COSMIN outcomes and the quality of the clinimetric properties. Data Synthesis. Six different measurement instruments or methods were identified. Test-retest, interrater, and intrarater reliability were investigated. Two test-retest reliability studies were rated as "fair" for the level of evidence. All other studies were rated as "unknown" for the level of evidence. Limitations. The paucity of literature describing clinimetric properties, especially other than reliability, of upper limb strength measurement instruments for children with CP was a limitation of the study. Conclusions. For measuring grip strength, the Jamar dynamometer is recommended. For other muscle groups, handheld dynamometry is recommended. Manual muscle testing (MMT) can be used in case of limited (below MMT grade 4) wrist strength or for total upper limb muscle strength. Based on lacking information regarding other clinimetric properties, caution is advised regarding interpretation of the results.