The study of the atmospheric composition and evolution of rocky planet atmospheres is key to understanding both the conditions required to develop a habitable planet, and to analyse the link between the deep interior and atmosphere of rocky bodies. This thesis uses volcanism as a chemical link between the mantle of a planet and its atmosphere, with the aim of analysing how a volcanically derived or supplemented atmosphere may appear, both under the end-member case where volcanism is the only factor affecting the atmosphere, and when changing surface temperatures and atmospheric escape is considered. Chapter 2 describes a newly developed model of volcanic degassing for COHSN elements, designed with the broad range of exoplanet geochemistry in mind. It also describes a model for simulating the evolution of a volcanic atmosphere through time, based on the initial volatile content of a planetary mantle, the surface temperature and a stipulation for the escape of hydrogen. Chapter 3 demonstrates that volcanic activity can sustain a fraction of hydrogen in planetary atmospheres undergoing hydrogen escape, which may have contributed to a cold, wet early Mars, and expands the liquid water habitable zone for exoplanets. Chapter 4 shows that on planets with Venus-like atmospheric temperatures, the mantle fO2 of a planet can be inferred from the chemistry and composition of a volcanic atmosphere as three distinct classes (defined by the presence/absence of certain indicator species) are formed. Specifically, Chapter 4 presents a set of volcanic atmospheres as an important base case for future research, exploring the effects of other processes on volcanic secondary atmospheres as produced by a range of geological conditions. Chapter 5 utilises chemical kinetics models to show that volcanic atmospheres must be at temperatures of 700K and above in order to be accurately modelled as in thermochemical equilibrium, with the reactions of key species (NH3, CO and CH4) being quenched over geological time below this point. Chapter 6 returns to the effect of hydrogen escape on volcanic atmospheres, exploring how escape modifies the atmospheric classes discussed in Chapter 4 and reduces or removes all indicators of mantle fO2 from the atmosphere. This thesis presents a new volcanic degassing model and a number of use-cases, demonstrating the wide range of chemical speciations which volcanically generated atmospheres can form.