Ocean acidification (OA) science has rapidly developed since 2005; however, international action remains limited. This chapter explains the complex scientific background of OA to non-scientists. OA is measured on a logarithmic pH scale, with oceans becoming 40 per cent more acidic since pre-industrial times. Three groups of compounds contribute to acidification: CO2 (the primary driver), nitrogen oxides (NOx), and sulphur oxides (SOx). These substances enter the oceans through various means, such as anthropogenic emissions, geoengineering, coastal activities, and scrubber effluents from ships. Additional ocean stressors, such as climate change, pollution, and overfishing, compound the effects of OA, making adaptation more challenging. OA threatens calcifying organisms such as corals and oysters, disrupts food webs, and impacts human ecosystem services valued at potentially $1 trillion annually by 2100. Adaptation options include blue carbon ecosystems, marine protected areas, and fisheries management. The issue encompasses ocean, atmosphere, and land systems across multiple timescales and spatial levels, necessitating diverse governance approaches that address both global CO2 emissions and local stressors.