The desire to monitor important neurotransmitters in the in vivo environment, in real-time and in conscious subjects has been the driving force behind the continued development over the last 40 years of a range of biosensor devices. This is a none too difficult task considering the milieu of substances that are present in vivo, particularly in the brain where there also exists a wide range of electroactive species, and where foreign objects are treated as hostile and subject to severe biological strain. Nevertheless, the rewards for developing a selective and sensitive biosensor are worth the effort. Today they are used, have been used and will increasingly be used for extremely important medical processes, including developing an understanding of disease etiology, determination of key intercession points in these pathologies, preclinical and clinical testing of proposed new treatments, and earlier diagnosis of medical conditions. With this in mind this thesis focused on the development and characterisation of a ᴅ-serine biosensor based around the flavin enzyme ᴅ-amino acid oxidase. In the recent past ᴅ-serine has been elevated from an “unnatural” amino acid to be recognised as a very important neurotransmitter that could be responsible for the regulation of a large portion of glutamate signalling in the forebrain. It has been highly implicated in a number a severe and widely occurring neurodegenerative diseases, ranging from schizophrenia to amyotrophic lateral sclerosis. Initial groundwork and development of a biosensor was underway when this thesis began. Extensive investigation and characterisation of this sensor, conducted at the start of this thesis, found that the biosensor displayed satisfactory sensitivity and selectivity properties. However, for the purposes of this project, to develop a biosensor suitable for chronic in vivo monitoring of ᴅ-serine, it was deemed unfit. This was due to an uneconomical and difficult to reproduce production methodology. Thus, from a solid starting point, from which much useful information had been gleamed, an entirely new biosensor was designed with the underlining principal of reproducibility and economic viability added to the need for sensitivity and selectivity. The final design involved the use of the cross-linking agent glutaraldehyde in conjunction with methyl methacrylate to immobilise the ᴅ-amino acid oxidase on to the surface of the electrode. To achieve effective interference rejection the dual use of Nafion® followed by an electropolymerised layer of poly-o-phenylenediamine was utilised. The electrode surface was a 125 μm Pt/Ir wire that was 0.5 mm in length. The design achieved a sensitivity of 16.47 ± 0.18 μA.cm-2.mM-1. It was found that the response was oxygen independent up to 100 μM ᴅ-serine. The limit of detection was determined to be 0.425 ± 0.005 μM and the biosensor has a response time of 5.95 ± 0.75 s. In the in vivo environment it was demonstrated that the biosensor could detect both increases and decreases in the endogenous concentration of ᴅ-serine, particularly in response to typical and traditional N-methyl-ᴅ-aspartate receptor antagonists like MK-801. Our device will make it possible to monitor, in vivo in real-time and without interference, the concentration of, and changes that occur in ᴅ-serine in a conscious subject. This could have a major impact on medical processes, both normal and pathological, whereby the metabolism of ᴅ-serine is of interest, including the development and testing of potential new drugs.