This work addresses the topic of so-called territorial externalities. After recalling the main definitions of this concept in Chapter I, an initial attempt is made to establish the taxonomy of the various externalities that may affect and identity a territory. Chapter II focuses on the role of "externalities of knowledge" in long-term economic growth processes. Chapter III highlights how, on the basis of current theoretical and empirical scientific contributions, no definite order of importance emerges between the dynamic externalities of specialisation or urbanisation that attempt to explain the growth and performance of certain territories. The consequences in terms of economic policies geared towards proactively exploiting the contribution of these territorial externalities are discussed in Chapter IV by highlighting one of the main challenges of the "social planner", i.e. that of considering the total net effects generated as a whole by the totality of the externalities that characterise a specific target area, as well as research into a "good social balance"