have been tried to improve the lactic acid production. The lactic acid bacteria, yeast and fungal systems have been engineered to enhance the lactic acid production. The advent of biotechnology and recognition of industrial applications of lactic acid led to the efforts being focused on use of biotechnological tools to engineer lactic acid bacteria (LAB) and other systems for the production of lactic acid. The initial efforts in LAB genetic modifications were concentrated mostly to develop LAB with enhanced qualities for food grade applications, using traditional approaches. The spontaneous mutations were also attempted by using insertion sequence (IS) elements. The LAB subjected to genetic improvement have been used in dairy industry for flavour enhancement, resistance to bacteriophages, addition of nutritional components and stability and structure of end products. The controlled gene expression systems for industrial gram-positive bacteria with low G + C content have already been reported. However, with the recognition of polylactide as a biodegradable polymer, attempts were directed to reduce the cost of lactic acid production by genetically modifying the organism, by using various cheaply available agro-industrial residues and by process modifications to remove the lactic acid produced during the course of fermentation. The authors here have tried to briefly summaries the various approaches to metabolic engineering used for improving the lactic acid production and cost reduction. (c) 2005 Elsevier Ltd. All rights reserved.