This chapter presents a case study rooted in the Jewish tradition of legalistic ethics, halakhah, discussing in particular its role in processes of conversion to Judaism. Jews-by-choice have become a prominent feature of the synagogues and Jewish organisations of contemporary Poland, the focus of the chapter. In the understanding of halakhah, conversion (giyur) means becoming an observant Jew. Converts willingly submit themselves to rules and constraints in order to effect a desired self-transformation. The institution of giyur creates the legal matrix that enables these individual, rule-oriented projects of the self and their recognition by the communities that the converts wish to join. However, the halakhah is characterised by pluralism. This form of self-making must thus be mediated through a multiplicity of rabbinical legal expectations – ‘progressive’, ‘orthodox’, ‘ultra-orthodox’ – and the varying degree of their recognition by a handful of especially important institutions, including, crucially for some, the Israeli state. In hierarchical terms, the strictest version of religious practice often becomes the most readily acknowledged. In practice, then, these transformative projects are realised not just through the rules of religious observance, but by navigating the meta-rules of conversion – which themselves vary – and the relationships, formal and informal, between these different legal matrices. Successful conversion thus often requires elaborate tactics of shifting between authorities, institutions and spheres of influence. An appreciation of its legalism thus becomes an indispensable technology of the self.