The ability of a simulation method to predict activation volumes reliably is valuable, because differences in activation volumes between two otherwise closely related Diels-Alder reactions are a sign of different reaction mechanisms. Hence, a tremendous amount of effort has been devoted to calculating reaction and activation volumes using various models. In an early work, for example, Monte Carlo simulation techniques were used on a periodic system to calculate reaction volumes of Diels-Alder reac-tions. Later, it was demonstrated that MD simulations are capable of reproducing experimental activation volumes (cf. also Section 2.3.2), e.g., in the dimerization reaction of cyclopentadiene. Recently, a QM/MM modeling approach allowed the calculation of reaction and activation volumes of the Diels-Alder reaction of cyclopentadiene and acrylonitrile. These studies demonstrate the power of today's computational tools and their competitiveness with experimental high-pressure techniques. The accurate calculation of reaction and activation volumes, e.g., of Diels-Alder reactions, opens up the possibility to screen a large variety of reactions at the computer and to identify the most promising candidates for later experimental verification. Nevertheless, studies on Diels-Alder reactions under pressure that intimately link experimental and computational techniques are still scarce. In a recent work, PBC-based metadynamics simulations of an azobenzene co-crystal have helped shed light on the reaction mechanisms of competing Diels-Alder ([4+2]) and [2+2] cycloaddition reactions. Moreover, Zholdassov and co-workers recently used a multiscale modeling approach to distinguish between uniaxial stress and hydrostatic compression in a sophisticated nanoreactor setup that induced Diels-Alder reactions between a surface-immobilized anthracene and different dienophiles. Such synergistic studies attest to the usefulness of high-pressure simulation techniques.