Name : Akinori Ohsawa Institute : Institute for Cosmic Ray Research E-mail : ohsawa@icrr.u-tokyo.ac.jp TITLE : Highest Energy Air Showers and Nuclear Interactions ABSTRACT : We will discuss how the characteristics of nuclear interactions, inelastic cross section, energy distribution of produced particles in multiple particle production, inelasticity, etc., affect the development of extremely high energy air showers. The study makes clear how large (or small) the ambiguity of energy determination is due to extrapolation of present knowledge on nuclear interactions. Based on the experimental data of multiple particle production by accelerator and cosmic-ray experiments, energy distribution of produced particles in multiple particle production is formulated. The formulated distribution shows violation of Feynman scaling law, and consequently resultant inelasticity decreases with the primary energy, amounting $< K > = 0.1$ at $E_{0}=10^{18}$ eV. We arrive at the conclusion that the energies of highest energy air showers which are estimated by assuming nuclear interaction models of QGSJET, VENUS, DPMJET, etc. have ambiguity of factor $\sim 5$.