  Graphene nanoribbons (GNRs) make up an extremely interesting class of
materials. On the one hand GNRs share many of the superlative properties of
graphene, while on the other hand they display an exceptional degree of
tunability of their optoelectronic properties. The presence or absence of
correlated low-dimensional magnetism, or of a widely tunable band gap, is
determined by the boundary conditions imposed by the width, crystallographic
symmetry and edge structure of the nanoribbons. In combination with additional
controllable parame-ters like the presence of heteroatoms, tailored strain, or
the formation of hetero-structures, the possibilities to shape the electronic
properties of GNRs according to our needs are fantastic. However, to really
benefit from that tunability and harness the opportunities offered by GNRs,
atomic precision is strictly required in their synthesis. This can be achieved
through an on-surface synthesis approach, in which one lets appropriately
designed precursor molecules to react in a selective way that ends up forming
GNRs. In this chapter we review the structure-property relations inherent to
GNRs, the synthesis approach and the ways in which the var-ied properties of
the resulting ribbons have been probed, finalizing with selected examples of
demonstrated GNR applications.
