The controlled growth of ultra-thin diamond layers on a diversity of substrates is a major challenge for many technological applications (heat spreaders, electromechanical systems, etc.). This explains the huge effort produced during the last two decades to master the early stages of diamond formation. Two main pathways have been investigated in the literature. The nucleation pathway aims to produce diamond nuclei, i.e., the smallest thermodynamically stable diamond islands, at the substrate surface. This is mainly performed by in situ treatments preceding diamond chemical vapor deposition (CVD) growth, such as bias enhanced nucleation (BEN). The second approach consists of skipping the nucleation stage by covering, ex situ, the substrate with diamond nanoparticles, which act as seeds for diamond CVD growth. The present chapter is a review of these pathways. Their respective benefits and drawbacks are discussed. Finally, these two approaches appear very complementary. Seeding allows the growth of ultra-thin diamond layers on large non-conductive substrates with micrometric patterns. On the other hand, the BEN in situ nucleation treatment remains the favored technique to achieve well-adherent diamond films and diamond heteroepitaxy.