The main objective of this thesis is to evaluate the thermal and mechanical behaviour of intumescent coatings (also known as reactive fire protection materials)that are used to protect steel structures from a fire exposure due to natural fires.As the intumescence process of currently available coatings deemed to be highlydependent on the heating regime, comprehensive experimental investigations areperformed to understand the correlation between the heating as well as the coolingregime and the thermomechanical performance of intumescent coatings. Dueto differences in sensitivity, a solvent-borne and a waterborne intumescent coatingare investigated by, among others, evaluating their insulation efficiency.Based on the experimental data, a thermomechanical material model is developed.This material model enables two- and three-dimensional finite element simulationsof the intumescent coatings applied on steel structures. As natural fires can showarbitrary regimes characterised by various heating and cooling rates, the materialmodel is formulated universally by taking the heating and cooling rate-dependentbehaviour of the intumescent coatings into account.Additionally to the finite element modelling, a MATLAB-based calculation approachis developed, aiming to reduce the computational effort and, thus, providinga design procedure for the dry film thickness of intumescent coatings for any desired fire exposure that is covered by the examined parameter range. As no otherapproach exists yet, this is the first scientifically-based method for the determination of the required dry film thickness of intumescent coatings to protect steel structures when being exposed to natural fires.