This thesis investigates the level of conformance of a selected list of computer-aided translation (CAT) tools and translation management systems (TMS) to the Core requirements of XLIFF (XML Localization Interchange File Format) version 2.1 through a series of tests based on six scenarios inspired by the translator's daily practice. The first part of this thesis provides an overview of localization and standards. The second part establishes the methodology: the material and theoretical framework for testing, the tools selection process and a step-by-step description of the tests conducted. The third part presents the results obtained from testing, also taking into consideration some of the most prominent tool features and functions that might raise issues when importing and translating an XLIFF 2.1 file. The results yielded by conducting conformance testing on 27 tools showed that only 9 tools can be considered compliant to the latest version of the standard, i.e. 33.3%.