During gonad development and fetal life, the germ cells (GC) undergo a range of different developmental processes necessary for correct postnatal gametogenesis and the production of the next generation. If these fetal events are disrupted by genetic or environmental factors, there could be severe consequences that may not present until adulthood. This is of particular importance in relation to human testicular GC tumours (TGCT), the most common cancer of young men, as TGCT is thought to arise from fetal GCs that have failed to differentiate normally during development and thus persist into adulthood, eventually becoming tumourigenic. TGCT is one of several related disorders of male reproductive health thought to comprise a Testicular Dysgenesis Syndrome (TDS), in which faulty testis development in fetal life may predispose to the development of cryptorchidism, hypospadias, reduced sperm count and TGCT. Currently there is no accepted animal model for TGCT, but some insight into human TDS has been gained through the use of a rat model using in utero Di (n-Butyl) Phthalate (DBP) exposure to induce cryptorchidism, hypospadias, low sperm count and reduced fertility (but not TGCT). However, a previous study suggested that DBP exposure can disrupt GC differentiation, resulting in significantly reduced GC number prior to birth and postnatal consequences. This thesis has been directed at investigating the normal process of GC development in the fetal rat and how this is altered by DBP exposure; such understanding may give insights into the origins of human TGCT by showing how and when disruption of normal fetal GC differentiation can occur. The first objective was to characterise GC development in both the rat testis and ovary to understand the normal events that occur between embryonic day (e)13.5 and e21.5, as most data in the literature is based on the mouse. Analysis by immunohistochemical, stereological and mRNA expression indentified that during this time period, a GC will undergo a dynamic sequence of changes involving migration, proliferation followed by differentiation (manifested by loss of specific protein markers), whilst undergoing germ-line specific remethylation. Whilst whole gonad development is vastly different between testes and ovaries, GC development was broadly the same with only minor differences up to the point where GCs in the ovary enter meiosis. Having established the normal process of GC development in the fetal rat testis, the effects of in utero DBP exposure was then investigated. DBP exposure reduced GC number at all ages investigated even after only 24 hours of exposure and simultaneously prolonged GC proliferation. As apoptosis was unaltered by DBP exposure, the consistent reduction in GC number was suggested to be due to an initial reduction in GC number that does not recover to control levels. GC differentiation was assessed by the expression and localization of specific protein markers (OCT4, DMRT1 and DAZL). The pattern of expression of OCT4 and DMRT1 was altered by DBP exposure. GCs in DBP exposed animals also showed a delay in disaggregation from within the centre of seminiferous cords. These results suggested that a delay in GC differentiation was occurring with DBP exposure. This delay in GC development persisted into early postnatal life, following cessation of DBP exposure. Thus at postnatal day (D)6, GC specific re-expression of DMRT1, GC migration to the basal lamina and resumption of GC proliferation all showed a delay. DBP also induced an increase in the presence of multinucleated gonocytes. DNA methylation in the fetal rat testis was also investigated as a mechanism that could be disrupted by DBP exposure. DNA methylation of GCs increased during the last week of fetal life by global methylation of the GC genome and the increased expression of DNA methyl transferases. No effect of DBP exposure was detected. Inhibition of methylation by 5-aza-2’deoxycytidine was then investigated as a way to block GC differentiation in fetal rat testes and this resulted in a similar transient delay in GC differentiation but was perinatally lethal to the fetus. Bisulphite sequencing of the OCT4 promoter was also performed but proved inconclusive. Methylation patterns may be being altered by DBP exposure, but such changes could not be identified in this thesis. To complement the in vivo DBP exposure studies, an in vitro testis explant system using e14.5 testes was investigated. These in vitro testis explants showed some GC effects with MBP, the active metabolite of DBP, and also suggested a novel role for Hedgehog signalling in GC survival in the fetal rat testis. The studies in this thesis have characterised several aspects of fetal GC development in the rat and identified which of these are affected by DBP exposure, resulting in a delay in GC development. As DBP exposure delays but does not block GC differentiation, this may explain why TGCT is not induced in the DBP exposure rat model for TDS.