Stoichiometry concerns the proportions of changes in the concentrations of substances participating in chemical or biochemical reactions. Since the stoichiometric structure of biochemical systems (i.e., the topology indicating which substances are linked with each other by reactions) can be considered constant in most cases, any modelling of such a system should begin with the analysis of its structure. In the present article, methods for deriving interesting conclusions about reaction systems from the information about stoichiometric coefficients are outlined. Firstly, this concerns conservation relations involving linear combinations of concentrations and the different possibilities of how substances are composed in terms of conserved moieties. Secondly, stoichiometric considerations enable the computation of flux distributions in stationary states. In particular, Metabolic Flux Analysis is discussed. In this analysis, experimental data on a limited number of reaction rates are used to compute the remaining fluxes in the system. Finally, we present a method for detecting elementary routes and cycles in the system. This method is a valuable tool for understanding the architecture of complex biochemical networks and for computing optimal stoichiometric yields of biotransformations.