Climate is one of many external drivers which have the potential to significantly influence water supply systems. However, a changing climate is the main driver to affect the availability of raw water resources. Predominately changes in the timing, frequency and intensity of precipitation can have a significant impact on the entire hydrological system. For example, increasing temperature often results in increasing water demand and enhanced water losses due to intensified evaporation. While increasing precipitation can damage water supply systems, due to flooding or increased erosion and water abstraction can be affected due to high turbidity and decreased water quality. Less precipitation or shifting seasonal precipitation events can cause serve water shortages, in particular when water storage facilities are insufficient. To respond to current and future climatic challenges and other external drivers, water resource management plans are developed and constantly upgraded. The traditional approach in water resource planning and management has been based on the assumption of stationarity of the hydrological system. However, the assumption that the past will be the key to the future is no longer valid. The climate and therefore the entire hydrological system is changing, and relying on the traditional planning approach increases the risk of mal-adaptation, water shortages and monetary losses. However, the methods to identify the future changes in water resources due to climate change are very uncertain. This cascade of uncertainty stems from the assumptions made about the state of future society and the greenhouse gas scenarios affiliated to these states. The uncertainty envelop expands further when the greenhouse gas scenarios are used to drive the global climate models, which are then downscaled to the regional models, and finally more uncertainty is added through local impact models. Such uncertain simulations are problematic when decisions on future adaptation have to be agreed upon to avoid expensive mal-adaptation. This chapter examines climate change as emerging pressure on water supply and the evidence of climate change from climate records. Future impacts of climate change on water resources are highlighted and the challenge of adaptation is reflected on. Being confronted with deep uncertainty, the need for alternative approaches, shifting the paradigm away from the traditional “predict and provide” approach, seeking an optimum adaptation solution, is highlighted. Instead, alternative approaches are needed that allow the development of water supply systems that are robust to the uncertainty framing future changes in water resources. International examples of these approaches are described; including a case study of the Boyne catchment's water supply system in the east of Ireland. The uncertainties involved in generation of future catchment hydrology are discussed and the vulnerability of the case study water supply system to a changing climate is investigated. Where vulnerability exists, sample plausible future adaptation options are then assessed to their robustness to uncertainty, to quantify the water supply system response, to aid decision making. This chapter concludes that water managers will have to engage with alternative methods for climate change adaptation in conjunction with future observational evidence of change.