Accurate cross section data for electron impact ionization (EII) are needed in order to interpret the spectra of collisionally ionized plasmas both in astrophysics and in the laboratory. Models and spectroscopic diagnostics of such plasmas rely on accurate ionization balance calculations, which depend, in turn, on the underlying rates for EII and electron-ion recombination. EII measurements have been carried out using the TSR storage ring located at the Max-Planck-Institut für Kernphysik in Heidelberg, Germany. Storage ring measurements are largely free of metastable contamination, resulting in unambiguous EII data, unlike what is encountered with other experimental geometries. As it is impractical to perform experiments for every ion, theory must provide the bulk of the necessary EII data. In order to guide theory, TSR experiments have focused on providing at least one measurement for every isoelectronic sequence. EII data have been measured for ions from 13 isoelectronic sequences: Li-like silicon and chlorine, Be-like sulfur, B-like magnesium, and F-like through K-like iron. These experimental results provide an important benchmark for EII theory.