Pressure changes many fundamental properties of atoms and challenges the way we think about basic chemical concepts. Examples include changes in atomic/molecular properties (Section 3.1.1), such as electronic configurations, atomic radii, electronegativity, ionization potentials and electron affinities, aromaticity and bonding patterns. Moreover, we will discuss pressure-induced spin crossover processes (Section 3.1.2) and changes in spectroscopic properties of compressed chemical systems (Section 3.1.3).