In business, there is a gradual transition from a simple interaction to an informal exchange, and then to agreement formalization as an exchange transaction. I examine the relative importance of vertical and horizontal interactions in market and non-market exchanges, as well as personalized versus depersonalized interactions. Unilateralism, as well as partial or total anonymity, generates relational forms, in particular opportunistic unilateralism and normative unilateralism, as well as their combinations. This chapter introduces transactions and contractual arrangements.