The primary aim of radiotherapy is to deliver sufficient dose to eradicate a tumour whilst sparing normal tissue. This balance in tumour control probability (TCP) and normal tissue complication probability (NTCP) can be influenced by a number of factors, one of which is respiratory motion. This thesis investigates the potential dosimetric and radiobiological differences as a result of respiratory motion in lung cancer radiotherapy. It demonstrates significant dosimetric improvements by using advanced motion management techniques (4DCT and respiratory gating) and modulated radiotherapy (VMAT), in regi~ns of lung tumour motion. These techniques confer improvements in NTCP and can allow for dose escalation. However, when patient characteristics and tumour characteristics are included in clinical modelling algorithms, the potential gain may not be as clinically relevant as anticipated. Many advanced techniques currently used in radiotherapy departments, have been implemented without a clear understanding of potential differences in radiobiological response compared with previous techniques. This thesis demonstrates significant differences in the radiation induced bystander effect (RISE), in the presence of respiratory motion and modulated radiotherapy. In vitro studies of respiratory gating indicate a trend towards increased survival as treatment delivery time increases. There is an increased dependence on the use of these radiotherapy techniques which introduce complex spatiotemporal dose modulation. The data presented in this thesis indicates that radiobiological consequences may not be fully explained by existing theories. These findings may be of particular relevance for modulated radiotherapy in NSCLC radiotherapy.