Cannabis is probably best known for one secondary compound, the psychotropic substance tetrahydrocannabinol (THC). Depending on the THC content of the plant or more specifically the dried inflorescence, Cannabis is either classified as marijuana (or drug-type, plants above 0.3% THC) or hemp (fibre-type, below 0.3% THC), which is mainly a legal and not a strict taxonomic classification. A more refined classification of Cannabis according to the phytocannabinoid profile into distinct ’chemotypes’ can also be useful, with chemotype I and II being marijuana while chemotypes III, IV and V can be seen as hemp (see chapter 3).
Many countries have been easing the ban on medical and even recreational use of THC during the past decade. However, because of the prohibition of Cannabis in many countries throughout the last century, it was not bred to the same extent as other high-value crops. Hence, hemp and marijuana lines retain a high level of genetic variability and heterozygosity, that is not found in other crops (Sawler et al., 2015).
Here, we review the biology as well as the applications and future perspectives of Cannabis research and breeding. We discuss Cannabis taxonomy and cannabinoid synthesis as well as flower development and flowering time control with an emphasis on sex determination in this predominantly dioecious species. We also summarize the currently available genomics resources. Since Cannabis is so versatile, we discuss its applications in medicine as well as in the building industry. Cannabis ’ future role in a sustainable society is summarized as well as the future of cannabinoid production via cell suspension cultures.

2. Cannabis systematics

Cannabis is the botanical name of a genus that historically includes three species, C. sativa, C. ruderalis andC. indica. However, since the three species can intercross, they are also often considered one single species, C. sativa (Small, 2015). Recent genetic data support the single-species concept and recommend that three subspecies should be recognized: Cannabis sativa subsp. sativa, subsp. indica and subsp. ruderalis (Q. Zhang et al., 2018).
Cannabis is a dioecious species, meaning there are male and female individuals (Figure 2a-c). However, through breeding, monoecious lines with male and female flowers on the same plant have also been generated (Figure 2d) (Moliterni et al., 2004).