Leptospirosis, a zoonoses with a worldwide distribution, is an acute febrile illness caused by spirochaetes of the pathogenic Leptospira. It affects a wide range of hosts including human, domestic and wild animal species. It is mostly present in tropical countries. The disease is often misdiagnosed as influenza, aseptic meningitis, encephalitis, dengue fever, hepatitis, or gastroenteritis. In domestic animals it is a disease of major economic importance as it affects productivity and results in weakness, abortion or even death. Timely diagnosis is essential because prompt, specific treatment, as early as possible is important to ensure a favourable clinical outcome. Laboratory confirmation is of utmost importance and until now serology has been the cornerstone of different laboratory diagnosis. In this book, we have described the recombinant antigen - based serologic tests which are required to achieve high sensitivity and specificity. Lip L32, a major leptospiral outer membrane protein has been identified as a sero-diagnostic marker for screening pathogenic leptospiral infection and as a recombinant fusion protein in a form suitable for diagnostic format.