In this paper we investigate the logic of speech acts and groundedness. A piece of information is grounded for a group of agents if it is publicly expressed and established by all agents of the group. Our concept of groundedness is founded on the expression of the sincerity condition of speech act theory. We formalize groundedness within an extended BDI (Belief, Desire, Intention) logic where belief is viewed as a kind of group belief. We show that our logic permits to reconcile the mentalist approaches on the one hand, and the structural and social approaches on the other, which are the two rival research programs in the formalization of agent interaction. Although groundedness is thus linked to the standard mental attitude of belief, it is immune to the critiques that have been put forward against the mentalist approaches, viz. that they require too strong hypotheses about the agents' mental states such as sincerity and cooperation: just as the structural approaches, groundedness only bears on the public aspect of communication. In our extended BDI logic we study communication between heterogeneous agents. We characterize inform and request speech acts in terms of preconditions and effects. We demonstrate the power of our solution by means of two examples. First, we revisit the well-known FIPA Contract Net Protocol. As a second example, we show how Walton & Krabbe's commitments can be redefined in term of groundedness.