Giardia is an enteric protozoan that infects a wide range of vertebrate hosts, being considered a major causative agent of gastrointestinal disease in humans in both developing and developed countries. Giardia infection has also a significant impact on livestock health, causing diarrhoea and resulting in significant economic lost. Transmission is either direct, through the faecal-oral route, or indirect, through ingestion of contaminated water or food. In this article, we review current knowledge about the epidemiology of giardiasis in different populations in Spain, including humans, livestock, companion animals, and wildlife. Environmental contamination of surface waters and raw foods by Giardia cysts is also addressed. Special relevance has been given to the data available on the molecular characterization of the Giardia isolates obtained from clinical and environmental samples, as determining thespecies/assemblages and subassemblages involved is essential for accurately identifying the parasite and assessing zoonotic transmission. The public health significance of these findings has also been thoroughly discussed. Giardia is an enteric protozoan that infects a wide range of vertebrate hosts, being considered a major causative agent of gastrointestinal disease in humans in both developing and developing countries. Giardia infection has also a significant impact on livestock health, causing diarrhea and resulting in significant economic lost. Transmission is either direct, through the faecal-oral route, or indirect, through ingestion of contaminated water or food. In this article, we review current knowledge about the epidemiology of giardiasis in different populations in Spain, including humans, livestock, companion animals, and wildlife. Environmental contamination of surface waters and raw foods by Giardia cysts is also addressed. Special relevance has been given to the data available on the molecular characterization of the Giardia isolates obtained from clinical and environmental samples, as determining thespecies / assemblages and subassemblages involved is essential for accurately identifying the parasite and assessing zoonotic transmission. The public health significance of these findings has also been thoroughly discussed.