External beam radiotherapy (RT) is the primary treatment modality for patients with inoperable lung tumours. Respiration-induced motion and related intra-/interfractional variations present a series of limitations to the success of existing conventional treatment modalities for lung cancer. Subsequently, to minimise the effects of respiration different management techniques have been proposed and are available. Respiratory gated radiotherapy (RGRT) holds promise to improve dose conformity, reduce the normal tissue control probability while increasing the tumour control probability. Its effectiveness depends on precise tumour localisation and targeting during dose delivery. In this thesis, the suitability of RGRT for the compensation of breathing induced motion was investigated by means of phantom studies and film dosimetry. Both regular and irregular trajectories were simulated during gated dose delivery and their effects on dose distributions analysed. Respiration-induced motion led to dose blurring and hence to less conformal dose distributions, which resulted overall in underdose of the treatment planning volume and an overdose of healthy surrounding tissue. Compared to non-gated dose delivery, RGRT improved dose conformity by enabling steeper dose gradients, resulting in an increased sparing of healthy tissue, at the expenses of increased delivery times. In the presence of irregular motion paths the dosimetric advantages of RGRT were observed to decrease. In the absence of a clinical tool for treatment verification such irregularities may pass unnoticeable and may lead to poor treatment outcomes. Investigations of the suitability of a software tool for tracking lung tumours in portal images during RGRT demonstrated that it is possible to determine and track tumour motion during gated treatment. Both the residual tumour motion inside the gating window as well as the probability density function were used as measures to quantify tumour position and variability. Tracking information was sufficient to quantify residual motion and variability. Baseline drifts as well as sudden fluctuations in tumour positions were detected and quantified, which led to considerable variations in residual motion which in turn may result in marginal miss. Although this was a retrospective analysis of motion data, the tool showed a great potential for verification of the tumour position during RGRT and may possibly be useful for adaptation of the gating window.