Creating or connecting together large organic molecules, as polycyclic aromatic hydrocarbons (PAH), by chemical reactions readily on surfaces is the first step to a true advance in the field of molecular electronics. On-surface synthesis can be regarded as an efficient means to build new molecular species by using bottom-up strategies. Recently, a collection of different reactions leading to large tailor-made organic molecules on single-crystal metal surfaces has been reported. The fundamental mechanisms controlling these reactions can be investigated from a surface science perspective. This discipline skillfully combines the use of characterization techniques at the nanoscale, with single-crystal metallic surfaces able to catalyse these reactions. We present a tutorial review that highlights the relevance of the new bottom up strategies and classifies most of the different molecular on-surface reactions involving aromatic organic molecules that have been published up to date.