This chapter gives an overview of the different spin‐state crossing scenarios affecting the reactivity of organometallic compounds. It focuses on the effects of crossing spin states in a number of elementary reactions typically observed for organometallic compounds, such as ligand exchange, oxidative addition, reductive elimination, migration/insertion, β‐hydride elimination, C‐H bond activation and intramolecular electron transfer between a transition metal and a coordinated redox active ligand. These processes are highly relevant for catalysis. To address the influence of the spin‐crossing on the reaction rate, Schroder et al. introduced the two‐state reactivity (TSR) concept, which applies if the overall kinetics for product formation results from an interplay between the barriers for the spin‐inversion and the barriers on the high‐ and low‐spin surfaces. The chapter discusses examples of homogeneous first‐row transition metal systems that catalyze (cyclo)oligomerization reactions of alkynes and alkenes, and which undergo a spin‐crossing during the catalytic turnover.