The results of electron spin resonance experiments on a series of transition metal ions in single crystals of corundum are reviewed and interpreted in terms of crystal field theory emphasizing three topics: (1) The strength of the cubic crystal field, (2) the axial field as an indicator of position of the impurity ion, and (3) Jahn‐Teller effects in the ground state. The 4dand 5dions which were examined, i.e., Ru3+, Rh3+, and Ir3+are found to be strong crystal field cases. In addition, the increased charge of Ni3+(3d7) is shown to be sufficient to drive it to a strong crystal field case (t26e) withS= ½ compared to Co2+(3d7) which ist25e3withS=32. The parametervwhich measures the trigonal field potential, was found to be +850 cm−1in Ru3+in good agreement with other ions such as Co2+and V3+whose cubic field ground states are also orbitally degenerate. In contrast, the determination ofvfrom ESR data on ions with singlet ground states such as Cu3+and Ni2+is regarded as less reliable. The Jahn‐Teller effect was observed for Pt+(or Pt3+), Ni3+, and Cu2+.