Since SHo Tome and Principe were colonized by Europeans in the 1470s alien mammal species have been introduced to these islands. The impact of these introduced species (14 to SHo Tome Principe and 12 to Principe) is unknown and though this paper reviews all the available information for each alien species no firm conclusions can be gleaned about their effect on the native biota of the Republica Democratica de SLo Tome e Principe. Despite the probable long association of many, if not all, the alien mammals it is clear that the long history of deforestation and habitat modification has had a greater detrimental impact to date. Although all the Red Data Book species are still present it is possible that the introduced mammals could cause future population declines and extinctions. Research and surveys are urgently required to discover the current status, distribution and ecology of both the native Red Data Book species and the alien species. These surveys, in conjunction with the identification of core areas for conservation would allow active management to control any detrimental impacts likely to be caused by the alien mammals. Once identified, regular monitoring programmes should be undertaken to ensure that the conservation aims are being achieved. At present there is a lack of any really suitable institutional organization with the responsibility or skills to undertake the required work. The development of such an organization should be a priority and support should be given both in the short and long terms by international organizations such as Birdlife International and the Gulf of Guinea Conservation Group.