The climate issue is becoming increasingly relevant, and with that, the EU's greentaxonomy has been designed to support investors in defining environmentally sustainableinvestments. To meet the taxonomy, the business must contribute significantly to at leastone of the six environmental objectives, not significantly harm the remainingenvironmental objectives, and meet minimum requirements for social sustainability.  Environmental certification systems are used to measure a building's environmentalsustainability, but as the taxonomy becomes more relevant, the question arises of howthese systems can be used in taxonomy work or if they become irrelevant. In addition,industry professionals experience the requirements of the taxonomy as unclear. Thepurpose of the report is therefore to analyze difficulties, develop solutions, and investigatehow environmental certification systems for existing buildings relate to the taxonomy.Miljöbyggnad iDrift, BREEAM In-Use, and LEED O + M have been investigated using aliterature review, interviews, and a case study. The main challenges with the EU-taxonomy were found to be the large margin forinterpretation left to affected organizations, due to a lack of clarifications. Another centraldifficulty was that the taxonomy's purpose as a common language fails, partly due tocountries using different systems to measure the sustainability of a building, making itdifficult to compare sustainability on a global scale. Proposed solutions included acommon certification system, demonstrating green potential, and authorities needing totake greater responsibility in providing assistance. An important conclusion of the thesisis that buildings that are certified with the analyzed systems of this report are notautomatically taxonomy-aligned, even though these systems play a continued significantrole in the sustainability work.