How does one structure, plan and cost a creative process that has inherent unstructured elements? Concept Development & Experimentation (CD&E) is a widely used term and methodology assisting transformation processes, research, and development. CD&E is such a creative process where a concept is developed through brainstorms, evaluation sessions and analyses combined with input from experiments. It leads to a robust concept that has been tried and tested in simulated experimental and operational settings, exposing aspects that could have been overlooked without experiments. But exactly this creative element brings uncertainty at the start of a CD&E process in terms of duration, cost and effort. TNO Defence, Security and Safety has developed Concept Maturity Levels (CMLs) in order to provide a framework to structure the CD&E process, to communicate its status and progress, and to estimate the required level of effort and duration. We have worked with CMLs for more than a year now and they have proven very effective, for both internal and external communication about CD&E projects. In CML a six-level scale defines the maturity of a concept, providing structure, activities per level, and enabling the definition of a concept development roadmap at the beginning of the CD&E process.