Mastitis, inflammation of the mammary glands of dairy cows, remains one of the costliest diseases in cattle and its early detection remains a major goal of the dairy industry. It has been established that mastitis causes changes in the conductivity of milk, by damaging the mammary membrane leading to alteration of the balance of sodium, potassium and chloride ions. Previous studies have demonstrated that these changes in milk conductivity can be used as a direct indicator o f the presence and degree of mastitis infection but currently available commercial devices do not appear to be sensitive enough to detect mastitis accurately on-line. The objective of this project is to design a novel electrochemical sensor to diagnose mastitis on-line in milking systems for cow herds. The study includes some preliminary experiments to assess the feasibility of developing such a sensor, the fabrication of the sensing system including the probe, the instrumentation and related software and the investigation of integrating the system into milking equipment. A number of primary experiments were carried out in order to investigate what parameters can be used to develop the sensor based on conductivity or impedance measurement. Milk properties such as dielectric properties, conductivity and oxidation behaviour have been investigated using various electrochemical techniques. Cyclic voltammetry was used to characterise the milk initially, electrochemical impedance spectroscopy was carried out to establish the specific electrical properties of milk and basic conductivity measurements were done to calculate the overall conductivity of milk. These methods were cross-referenced against Somatic Cell Count (SCC) measurements which were determined using a bench top instrument in Teagasc National Centre Research laboratories. Electrochemical impedance measurements showed that measurements should be taken above an operating threshold frequency to avoid polarisation effects and to improve the sensitivity o f the conductivity sensor. Above this frequency, a good correlation between Somatic Cell Count (SCC) and impedance in milk has been found.