The principal aims of this project are to develop an understanding of colour and constitution relationships and the factors which influence the fluorescence properties in benzoxazinone-based fluorescent dyes, aza analogues of coumarin dyes, to explore the synthesis of a range of these benzoxazinone-based fluorescent dyes and to investigate their UV/visible and fluorescent spectral properties. A particular focus of this investigation was optimized of the synthesis of the dyes and the effect of solvents and other environmental influences on the electronic absorption and emission spectra of the dyes which might be usefully explored to give biological probe molecules. Molecular modeling studies using the CAChe system (AMI, MM2 PM3, ZINDO calculations) and PPP-MO calculations were used to investigate relationships between the chemical structure, spectral properties and technical performance. Reasonable spectral correlations were found using PPP-MO calculations to predict the colour of the benzoxazinone dyes. Weak solid-state fluorescence was observed under UV-light for the benzoxazinone dyes. This interesting feature was investigated in two cases by x-ray crystallography to explore the reasons for this observation. A test for cytotoxicity was carried out on benzoxazinone and coumarin dyes by the ISO 10993-5 method, which showed slight toxicity for the dyes. On the basis of the results of the investigation it was concluded that the benzoxazinone dyes have potential as probe molecules for biological application. Textile dyeing procedures using the dyes as disperse dyes for polyester were optimized. Colour measurement and assessment of fastness properties were carried out on the dyed fabrics.