Hexagonal boron nitride (hBN) monolayers have attracted considerable interest as atomically thin sp2-hybridized sheets that are readily synthesized on various metal supports. They complement the library of two-dimensional materials including graphene and open perspectives for van der Waals heterostructures. In this review, we discuss the surface science of hBN including its growth, the hBN/metal interface and its application as template for adsorbates. We mainly focus on experimental studies on hBN/metal single crystals under ultra-high vacuum conditions. The interfaces are classified regarding their geometric structure - ranging from planar to strongly corrugated overlayers - and their electronic properties - covering weakly and strongly interacting systems. The main part of this review deals with hBN/metal substrates acting as supports for adsorbates such as individual atoms, metal clusters, organic molecules, metal-organic complexes and networks. We summarize recent surface science studies that reveal the unique role of the hBN/metal interfaces in tailoring characteristic properties of such adsorbates. Central aspects include templating and self-assembly, catalytic activity and on-surface reactions, electronic and magnetic structure. As many of the resulting systems feature superstructures with periodicities in the nanometer range, a length scale also reflecting the size of adsorbates, scanning probe microscopy is one of the most common techniques employed. In short, the goal of this review is to give an overview on the experimental and complementary theoretical studies on hBN templates available to date and to highlight future perspectives.