Drinking water, also known as potable water, should be risk free of any health concerns. Contaminated water with fecal and high doses of undesirable pathogens and chemicals can be source of many waterborne diseases, such as typhoid, diarrhea, cholera, and hepatitis A. Access to safe drinking water is the key to prevent the spread of any waterborne diseases. Out of 190 countries in the world, only 61 countries have the 99% of their population with access to safe drinking water. In 19 countries, 50% of the rural population do not have access to safe drinking water. Analysis shows that percent of population in a country with access to safe drinking water increases with the increase of gross domestic product (GDP) up to US $10,000. Most of the countries with at least 1000 diarrheal death per year are found to have lower GDP. Climate change which simply means shifting weather patterns including increasing temporal and spatial variabilities of temperature and precipitation has great impact on water supply. The predictability of surface water availability becomes more uncertain due to climate change. While the water demand is expected to rise about 25% in 2050, the climate change can affect streamflow due to increase in temperature and decrease in precipitation. Additionally, the number of watersheds that is difficult to predict will increase significantly due to climate change. The sensitivity of water supply system to temperature and precipitation and the system response can vary location to location. In a case study in California, it is found that while one water supply system needs 10% increase in precipitation to offset the impact of 20 °C temperature increase, another water supply system needs 20% of precipitation increase to offset the impact of 20 °C temperature increase. System response decreases with the increase of temperature. Water supply response can decrease up to 40% at 40 °C temperature increase. With currently 21 countries facing extremely high water stress, analysis shows that the projected number of countries that will be facing at least high water stress is 51 in 2050. Most of these countries are in the Middle East, North Africa, Europe, and Central Asia. It is found that mortality rate is generally inversely proportional to the percent of population that have access to safe drinking water.