We present the phase diagram of the mean-field driven-dissipative Bose-Hubbard dimer model. For a dimer with repulsive on-site interactions (U>0) and coherent driving, we prove that 2 -symmetry breaking, via pitchfork bifurcations with sizable extensions of the asymmetric solutions, require a negative tunneling parameter (J<0). In addition, we show that the model exhibits deterministic dissipative chaos. The chaotic attractor emerges from a Shilnikov mechanism of a periodic orbit born in a Hopf bifurcation and, depending on its symmetry properties, it is either localized or not.The Bose-Hubbard model is a celebrated fundamental quantum mechanical model that accounts for boson dynamics in a lattice [1]. It successfully describes the interplay between the hopping of particles between neighboring sites of the lattice (with rate J) and on-site interactions. Such interactions appear as multi-boson terms in the Hamiltonian with interaction energy U. Importantly, this model accurately explains the superfluid to Mott insulator phase transitions, that has been experimentally demonstrated in ultracold atomic lattices [2]. A minimal building block in this context is the so-called Bose-Hubbard dimer, consisting of only two interacting sites, also known as the bosonic Josephson junction [3,4]. Furthermore, the Bose-Hubbard dimer lies at the basis of a number of striking phenomena such as the Josephson effect [5], self-trapping [6] and symmetry breaking [7].In recent years there has been a growing interest in understanding open quantum systems, where the bosons can be added and destroyed by means of external driving and dissipation mechanisms [8][9][10][11]. In this context, photonic systems have attracted much attention since photons in optical cavities can be injected through an external driving laser, and dissipation comes in as a natural consequence of optical cavity losses. The drivendissipative Bose-Hubbard dimer has been realized in a number of experimental systems, such as semiconductor microcavities [12,13], superconducting circuits [14] and photonic crystals [15], where the interactions take the form of Kerr-type optical nonlinearities. In many regards, these optical systems constitute outstanding platforms for studying many-body phenomena in open quantum systems [16]. Among them, dissipative phase transitions [17,18] are an especially exciting open topic, because they provide a conceptual basis for the understanding and the prediction of new collective states, both steady and dynamical ones, with the latter accounting for collective coherent oscillations.As is well known, phase transitions are characterized by critical phenomena, which can emerge in the thermodynamic limit, i.e. with a large photon number in optical cavities. Remarkably, even single-mode cavities -i.e. with no spatial degrees of freedom-may display phase transitions including optical bistability (which is of first order) [18,19] and the emergence of an oscillation threshold in, e.g. two-photon pumped Kerr resonators [20] or laser devices [21] ...