Leishmaniasis is a complex vector-borne disease caused by Leishmania sp. (Kinetoplastida: Trypanosomatidae) and is transmitted by female sand flies (Diptera: Psychodidae). It is endemic in 88 countries where the prevalence is 12 million and 350 million people are at risk. The study of vector biology is essential to understand leishmaniasis epidemiology and to implement cost-effective vector control measures. This thesis aims to describe the role of Promastigote Secretory Gel (PSG) in disease transmission. Transmission is a crucial event in the Leishmania life cycle, requiring sufficient infective metacyclic promastigotes to be injected in the host by sand fly bite in a favourable ecotope. PSG is produced by immature Leishmania forms within the female sand fly gut and it is comprised of filamentous proteophosphoglycan (fPPG). It is known to modify sand fly behaviour. Here, we investigate its biological role in development and enhancement of transmission by parasite selection using a Lutzomyia longipalpis-Leishmania mexicana experimental model. Briefly, a Lutzomyia longipalpis colony was established and maintained at LSHTM and all parasite developmental forms were generated. PSG was obtained from experimentally infected flies. PSG role in development, specifically in attachment and detachment of promastigotes to the midgut, was studied by ex vivo competitive midgut binding. The role of PSG in parasite selection was studied by in vitro PSG slide attachment, capillary migration and parasite video-tracking. In order to study the underlying binding mechanisms to both PSG and midgut, mutant parasites were used. Ligand analysis was completed by Western Blotting. PSG was found to favour promastigote detachment from the midgut and thus, parasite migration and maturation. It binds to immature parasitic forms and not to infective metacyclic promastigotes, creating a biological sieve that retains immature forms and enriches the bite for metacyclics. Collectively, the results show that PSG is an essential vector competence determinant allowing successful transmission.