The bound system of top and antitop quarks, called toponium, can be used for spectroscopic studies at distance scales of less than 10$^{-14}$ cm. Physics at such short distance scales is an abundant source of new information. The possibilities of creation of new particles in toponium decays, the determination of quark structure from spectroscopic data on toponium, and the verification of basic ideas on strong and electroweak interactions are reviewed.