Quantum chemical calculations have today found widespread applications. In my presentation I want to illustrate this with examples from my own research focusing on the interplay between calculations and experiment. The examples will range from perturbed angular correlation (PAC), nuclear magnetic resonance (NMR) or electron paramagnetic resonance (EPR) spectroscopy studies on transition metal complexes, over NMR studies of various organic compounds to UV/Vis spectra of organic dyes and the calculation of rate constants for the reactions of OH-radicals with DNA nucleobases.