Recent evaluations of the knowledge of biodiversity report that marine species account for only 15% of the total known taxa. The lack of knowledge on marine diversity is coupled by an accelerating rate of diversity loss, due mainly to human factors. The purpose of this review is to present a summary of the main anthropogenic threats to marine biodiversity, such as resource overexploitation, nutrient loads, oil spills, habitat destruction, invasive species, harmful algal blooms, and global climate change. These threats may act together, and their impact on marine ecosystems is still largely overlooked and not completely understood.