Cystic Fibrosis (CF) is one of the most common genetic diseases affecting Caucasians that leads to early deathi. It is Ireland’s most common life- threatening inherited disease. Early diagnosis facilitates early implementation of therapy, which can significantly improve the prognosis and life quality of the affected patient. The gold standard diagnostic test for CF is the Quantitative Pilocarpine Iontophoresis Test (QPIT) for the sweat electrolytes sodium and chlorideii. In the QPIT, the sweat glands are artificially stimulated, a sweat sample is collected, and the concentrations of sodium and chloride ions determined separately using standard benchtop analytical instrumentation. In this thesis, the use of miniature solid-state Ion-Selective Electrodes (ISE’s) to monitor both target ions simultaneously, for the purpose of CF diagnosis, is investigated. A commercially available sensor array in a flow-cell was initially used in both simulated and live clinical trials, involving the analysis of real sweat samples from confirmed CF and non CF populationsiii. Solid-state ISE’s were then designed, constructed, tested and incorporated into a bespoke flow-cell device compatible with modern sweat sampling equipment. Finally a working prototype of a wearable, watch-type device for the rapid on-line diagnosis of (CF) was designed, constructed and employed in a live clinical trial to assess its usefulness as a possible real-world diagnostic / screening device.