**1. Introduction**

Despite the early pioneering observations dating back to the nineteenth century [1], a settled prejudice that pathogens basically were the only microorganisms able to colonize living plant tissues has long delayed the awareness that endophytic fungi are constantly associated to plants, and remarkably influence their ecological fitness. Overcoming an apparent vagueness of the concept of 'endophyte', scientists working in the field have agreed on the opportunity of delimiting what belongs to this functional category. Thus, a series of definitions have been enunciated which are all based on the condition of not causing any immediate overt negative effect to the host [2].

Besides being prompted by the general theoretical intent that all components of biodiversity from natural contexts ought to be exploited for the benefit of humanity, investigations on the endophytic microbiota, or endosphere [3], have also been undertaken with reference to crops. In this respect, it can be said that endophytes are even more relevant in orchards, where the time factor confers higher impact to the establishment of an equilibrium among the species which are part of the tree biocoenosis, and to its possible disruption. Hence, all sorts of contributions have recently been proliferating in the literature, to such an extent that an organization of the available information is now appropriate in order to support the scientific community in achieving further progress. In view of this perspective, the present paper offers a review of the state-of-the-art of research concerning occurrence and effects of endophytic fungi associated with citrus species.
