A key to sustainable management of water resources is having the knowledge and scientific basis needed to make the right decisions. The application of isotopic techniques in Bangladesh over the last few years have proved valuable to improve our understanding on the behaviour of groundwater systems in the coastal areas. In the southern part of the country, groundwater salinization limits the supply of fresh water to the expanding population, irrigation system and industrial production. The young (Holocene) alluvial deposits in the southern part of deltaic basin are mostly affected with high arsenic water. The arsenic contamination (having highest concentration of 711 µg/L) crisis in the shallow aquifers of coastal area (Khulna, Satkhira, Bagerhat, Barisal, Patuakhali, Barguna, Chandpur and Lakshmipur) has engendered a wide variety of responses to the challenge of safe drinking water supply. Because deeper aquifers in this region generally have low arsenic concentrations they represent a potential source of safe drinking water. An assessment of the potential for development is needed, however. The challenges facing the national projects are to identify a source of safe drinking water and to protect it from degradation. The deep aquifers are subjected to intensive pumping causing large drawdown of piezometric head. It sometimes causes the dewatering of confined aquifer and reduces the production section of wells. A systematic environmental isotope (δ18O, δD, δ13C, 3H and 14C) including geochemical study was carried out to understand the groundwater flow system, sources of aquifer recharge and salinization, and connectivity between different water bodies. The study work was associated with Regional Cooperative Agreement (RCA) and national Technical Cooperation (TC) projects of International Atomic Energy Commission (IAEA). Based on the determination of the stable and radioactive isotope composition, these isotope hydrology projects give important results and form the basis for the decision of the future protection and exploitation of the aquifers. The δ18O compositions of shallow (<70m depth) and deep (>145m depth) groundwaters range from –7.50 to –2.19‰ and –4.44 to –1.87‰ VSMOW respectively. The stable isotope data indicates that all groundwater is derived from rainfalls and/or flood water. The stable isotopes of adjacent river waters are close to those of shallow groundwater suggesting mixing with river water may be occurring. Both the river and shallow groundwaters have high chloride concentrations associated with high EC values. The deep groundwater ages range from 8,000 to 24,000 years BP. In depth dependence plot of Carbon-14 values, groundwaters show relatively low pmC values (5.35 to 34.8 pmC) and these groundwaters appear to be the end member of the deep groundwater flow systems in Bengal Delta, where it mixes with the chloride rich (average 502 mg/L) water. This deep water salinity can possibly be attributed to contribution of salts from the marine connate water. The carbon-14 data further confirm the observation based on oxygen isotope values that these relatively fresh, but a few exception of high salinity in a few deep groundwaters have been recharged in different climatic regimes thousands of years ago.