The concept of 'additionality' in Clean Development Mechanism (CDM) projects is crucial for ensuring that emission reductions wouldn't have happened without the project. The definition and implementation of additionality have evolved over time, with the CDM Executive Board introducing additionality testing and tools. Recent developments allow for the use of benchmarks and default factors to assess project additionality. Some project types, like cement blending and waste heat recovery, have faced rejection based on additionality criteria. Proposals for reforming additionality testing have been suggested. Scrapping additionality testing could have significant implications for CDM projects. Lessons from five years of additionality determination have been summarized.