Although it has never been attempted in humans so far, germline gene therapy has been an object of ethical controversy for a long time already. The classical arguments in favour and against germline gene therapy will be briefly reviewed. Much of the vagueness and lack of focus of the debate stems from two conceptual problems: 1. Germline therapy on humans is a technology whose concrete empirical outlines can only be guessed at. Ethical evaluation of a technology which still belongs to a speculative future is beset with epistemological uncertainties, which raise the more general problem of anticipatory evaluation and regulation of 'exotic' biotechnologies. 2. It is not clear what sort of germline interventions would qualify as "therapy" (straightforward application to humans of today's animal transgenic techniques would almost certainly not). This is an added element of conceptual uncertainty that makes an ethical evaluation difficult. Nevertheless, I want to propose an ethical framework based on E. Juengst's distinction between phenotypic vs. genotypic prevention, as an alternative to the conventional view that relies heavily on the somatic vs. germline distinction.