The importance of chloride ions in reinforcement corrosion has led to the concept of a chloride threshold level, namely the minimum chloride content at the depth of steel that would result in the passive film breakdown. The reported chloride threshold level values range from 0.2 to more than 2% by weight cement. The physical condition of the steel-concrete interface appears to be the most important factor affecting the threshold level. The importance of the steel concrete interface is investigated in this research. Extensive experimental work was carried out. This involved the development of methods to create entrapped air voids at the interface, characterise the physical condition of the interface and quantify its effect on the chloride threshold level. A large number of specimens were cast and tested with variable interfacial conditions as well as various cement binders. Voids at the interface were best obtained by varying compaction and mix design, and quantified using an image analysis tool. For the first time chloride threshold levels ranging between 0.2% and 2.5% by weight of cement were reproduced in a single laboratory programme of work. The threshold level was observed to be strongly dependent on entrapped air content, confirming the importance of previously un-quantified interfacial conditions. It was that shown the reduction of percentage voids at the interface to less then 0.2% resulted in a sharp increase in chloride threshold level values to above 2% by weight of cement. The chloride threshold level could be raised by improving compaction, using an admixture and applying an inhibitive steel treatment or electrochemical treatment. In addition to quantifying the effect of the steel concrete interface on the chloride threshold level, new methods of characterising concrete were developed. A novel method was used to measure the chloride binding capacity and the acid neutralisation capacity test was refined to measure residual buffering capacity of concrete. A more appropriate index for presenting chloride threshold level than those currently used was suggested. This was based on the experimental data obtained that showed many of the hydrated solid phases of concrete inhibit corrosion initiation by resisting a fall in pH, while bound chloride presents a corrosion risk due to pH dependent dissolution characteristics of solid phases.