The importance of odd hydrogen chemistry was initially confined to the mesosphere because of the ineffectiveness of reaction as a source of HOx (H, OH, HO2) in the lower atmosphere. This chapter summarizes the uncertainties regarding the stratospheric HOx cycle and identifies outstanding problems that need to be resolved. It focuses on subsequent new developments in the area of odd hydrogen chemistry. The chapter discusses the salient features of the odd hydrogen problem, particularly those features which can be verified by future atmospheric measurements. Reactions involving recycling of odd hydrogen play an important role in stratospheric chemical processes controlling odd oxygen balance. Stratospheric H2O plays an important role in both chemical processes and in the global radiation budget. Changes in stratospheric ozone could conceivably affect the tropopause temperatures which limit the influx of H2O from the troposphere to the stratosphere.