The respiratory system is highly compromised after a tetraplegic spinal cord injury due to paralysis of the major breathing muscles. As a result mechanical ventilation is often required and respiratory complications are a major cause of rehospitilisation, morbidity, and early mortality. Functional electrical stimulation (FES) applied acutely to the abdominal wall muscles in synchrony with a patient's volitional exhalation has been shown to improve breathing volumes and the ability to cough in spontaneously breathing tetraplegic patients. It has also been used acutely to improve breathing volumes in otherwise mechanically ventilated patients. The effect of using abdominal FES (AFES) chronically on AFES-assisted and unassisted respiratory function is currently unknown. To support clinical adoption of AFES practical systems are required. Systems that synchronise AFES with exhalation automatically have been developed but they have relied on invasive respiratory sensors. In the first clinical study of this thesis twelve tetraplegic patients who could breathe spontaneously completed a three week AFES training programme in addition to a one week pre-training control period and a three week post-training follow up period. The results showed a significant increase in \ac{afes}-assisted forced vital capacity (FVC), and unassisted FVC, peak expiratory flow (PEF), and cough peak flow (CPF) throughout the training period. AFES-assisted PEF and CPF tended to increase over the same period, but the increase was not significant. The difference between unassisted and AFES-assisted measures did not change. Overall, there were limited changes in the outcome measures during the control and follow up periods, which suggests that the changes in outcome measures observed during the training period were a response to training. In the second clinical study daily sessions of AFES-breathing were combined with the standard of care during the process of weaning a single tetraplegic patient from mechanical ventilation. The results showed that the approach was feasible: AFES acutely increased the duration of ventilator free breathing at the start of the weaning process and daily ventilator free breathing improved considerably during two four-week long periods of daily AFES-assisted breathing. In the final study breathing data was recorded from ten healthy volunteers using a spirometer (the current standard), a nasal thermocouple, and piezoelectric belts wrapped around the chest and abdomen. An algorithm was written for each of the sensors so that they could be used to trigger stimulation during quiet breathing. The thermocouple system, followed by the chest belt system, were shown to be the most suitable replacement sensors for the spirometer. The results of this thesis suggest three different applications of AFES in tetraplegia: a neurorehabilitation device that can be used to improve unassisted respiratory function in spontaneously breathing tetraplegics; a neuroprosthesis device that could be used to assist spontaneously breathing tetraplegics in times of respiratory distress, e.g. during recovery from respiratory infection; and as a method of weaning tetraplegic patients from mechanical ventilation. The realisation of these applications will be assisted by the non-invasive respiratory sensor algorithms developed in this thesis. Collectively these results have demonstrated the feasibility of several new areas of future research, which could ultimately be of great benefit to the health of patients with tetraplegia.