Photoactive Cu(I) complexes are considered as a promising alternative to traditional noble-metal complexes that are mainly based on Ru(II) and Ir(III). Therefore, in recent years heteroleptic diimine-diphosphine [Cu(N^N)(P^P)]complexes have been developed exhibiting impressive performance, e.g. long-lived excited-states, high emission quantum yields with tunable emission spectra as well as largely adjustable redox properties. These favorable features render heteroleptic [Cu(N^N)(P^P)] complexes as suitable non-noble metal photosensitizers. Due to the exploding number of examples reported in literature, this review aims at systematically comparing the fundamental features of various [Cu(N^N)(P^P)] complexes regarding to their geometrical, optical and electrochemical properties. In particular, this tutorial review focuses on the ability to tune the optoelectronic properties and excited-state dynamics by the design and the choice of the underlying N^N and P^P ligands. Finally, this review briefly indicates some application scenarios of these novel class of complexes, e.g. in the fields of photocatalytic hydrogen evolution and light-emitting devices. Asa result this tutorial review likes to contribute to a detailed understanding of structure-function relationships and to the rational design of heteroleptic [Cu(N^N)(P^P)] complexes for specific photophysical applications in the future.