This special issue combines original research with seminal reviews of the biological impact of ocean acidification. The ten contributions cover a wide range of topics from chemical and biological responses to increased CO2 and decreased pH to socio-economical sensitivities and adaptation options. Overall, this special issue also highlights the key knowledge gaps and future challenges. These include the need to develop research strategy and experiments that factor in evolution, incorporate natural variability in physical conditions (e.g., pH, temperature, oxygen, food quality and quantity) and ecological interactions. The research presented in this special issue demonstrates the need to study more habitats (e.g., coastal, deep sea) and prioritize species of ecological or economic significance.