Ocean acidification is a process that occurs due to an increase in carbon dioxide concentration in the oceans, which leads to lower pH values. It largely depends on the concentration of carbon dioxide in the ocean, but it can be enhanced by other factors, such as the process of nitrification and temperature rise. Due to global warming, ocean acidification has particularly intrigued scientists and general public for the last few years. This work presents the overview of studies made about the impact of ocean acidification on some key groups of marine organisms. Because of the foreseen increase of pH concentration in the sea, a special attention was paid to the process of calcification in organisms that encrust calcium carbonate (plankton, corals, echinoderms, molluscs and crustaceans) and beside that, the problems that non-calcifying organisms face, such as fishes, were addressed. Macroalgae were also discussed. Scientists have noticed that different organisms react differently to ocean acidification, but they all agreed that, with the decrease of pH concentration in the oceans and world seas, the balance will be changed, and that will impact key chemical and biological processes. Man is one of the most important environmental modifiers. If the trend of increasing CO2 emission continues, it will cause serious disturbances in the marine ecosystem. Species that play important roles in food webs could disappear, the life and production of phytoplankton would be disrupted, there would be no more coral reefs, and all this would lead to irreversible changes in the marine environment and dramatic consequences for humanity.