Giardia sp. is a common intestinal parasite in humans, causing giardiasis, which is characterized by diarrhea, abdominal cramps, bloating, weight loss, and malabsorption. It is also prevalent in domestic and wild animals. Giardia duodenalis, a species within the genus, causes giardiasis in humans and many mammals, making it a zoonotic disease. The life cycle of Giardia is direct, with cysts becoming infectious upon release in feces. Cysts can survive for months in cool, damp environments and are spread through contaminated water, food, and fomites. Giardiasis has a significant public health impact due to its high prevalence, disease burden, and effects on growth and cognitive development in children. It is also a common disease in livestock and companion animals, highlighting its veterinary importance. Giardia duodenalis has a global distribution, with infection rates varying by region and population. In developed countries, infection rates in humans are generally lower, while in developing countries, they are higher. The disease burden is significant, with an estimated 2 million cases annually in the United States. Giardiasis can lead to malnutrition, growth delays, and cognitive impairments, particularly in children. Outbreaks are well-documented, often linked to waterborne transmission, and can affect both humans and animals. Giardia species include G. duodenalis, G. agilis, G. ardeae, G. muris, G. microti, and G. psittaci, with G. duodenalis having the broadest host range and greatest public health significance. Recent molecular studies have identified various assemblages and subassemblages of G. duodenalis, with assemblages A and B being zoonotic and having broad host specificity. Other assemblages, such as C, D, E, F, and G, have more restricted host ranges. Molecular diagnostic tools, including PCR and sequencing, are used to differentiate Giardia species, assemblages, and genotypes. These tools are essential for understanding the epidemiology of giardiasis, tracking outbreaks, and identifying infection sources. Population genetic studies suggest that Giardia may undergo genetic recombination, challenging previous assumptions of asexual reproduction. However, the exact mechanisms of reproduction and genetic diversity in Giardia remain unclear. The use of multilocus genotyping and other molecular tools is crucial for accurate identification and characterization of Giardia species and their role in zoonotic transmission. Understanding the genetic diversity and epidemiology of Giardia is essential for developing effective prevention and control strategies.Giardia sp. is a common intestinal parasite in humans, causing giardiasis, which is characterized by diarrhea, abdominal cramps, bloating, weight loss, and malabsorption. It is also prevalent in domestic and wild animals. Giardia duodenalis, a species within the genus, causes giardiasis in humans and many mammals, making it a zoonotic disease. The life cycle of Giardia is direct, with cysts becoming infectious upon release in feces. Cysts can survive for months in cool, damp environments and are spread through contaminated water, food, and fomites. Giardiasis has a significant public health impact due to its high prevalence, disease burden, and effects on growth and cognitive development in children. It is also a common disease in livestock and companion animals, highlighting its veterinary importance. Giardia duodenalis has a global distribution, with infection rates varying by region and population. In developed countries, infection rates in humans are generally lower, while in developing countries, they are higher. The disease burden is significant, with an estimated 2 million cases annually in the United States. Giardiasis can lead to malnutrition, growth delays, and cognitive impairments, particularly in children. Outbreaks are well-documented, often linked to waterborne transmission, and can affect both humans and animals. Giardia species include G. duodenalis, G. agilis, G. ardeae, G. muris, G. microti, and G. psittaci, with G. duodenalis having the broadest host range and greatest public health significance. Recent molecular studies have identified various assemblages and subassemblages of G. duodenalis, with assemblages A and B being zoonotic and having broad host specificity. Other assemblages, such as C, D, E, F, and G, have more restricted host ranges. Molecular diagnostic tools, including PCR and sequencing, are used to differentiate Giardia species, assemblages, and genotypes. These tools are essential for understanding the epidemiology of giardiasis, tracking outbreaks, and identifying infection sources. Population genetic studies suggest that Giardia may undergo genetic recombination, challenging previous assumptions of asexual reproduction. However, the exact mechanisms of reproduction and genetic diversity in Giardia remain unclear. The use of multilocus genotyping and other molecular tools is crucial for accurate identification and characterization of Giardia species and their role in zoonotic transmission. Understanding the genetic diversity and epidemiology of Giardia is essential for developing effective prevention and control strategies.