We derive the Bethe ansatz equations describing the complete spectrum of the transition matrix of the partially asymmetric exclusion process with the most general open boundary conditions. For totally asymmetric diffusion we calculate the spectral gap, which characterizes the approach to stationarity at large times. We observe boundary induced crossovers in and between massive, diffusive and KPZ scaling regimes. The partially asymmetric simple exclusion process (PASEP) describes the asymmetric diffusion of particles along a one-dimensional chain with L sites. It is one of the most studied models of non-equilibrium statistical mechanics, see [1,2] for recent reviews. This is in part due to the fact that is one of the simplest lattice gas models, but also because of its applicability to molecular diffusion in zeolites [3], bioploymers [4], traffic flow [5] and other one-dimensional complex systems [6].At large times the PASEP exhibits a relaxation towards a nonequilibrium stationary state. An interesting feature of the PASEP is the presence of boundary induced phase transitions [7]. In particular, in an open system with two boundaries at which particles are injected and extracted with given rates, the bulk behaviour in the stationary state is strongly dependent on the injection and extraction rates. Over the last decade many stationary state properties of the PASEP with open boundaries have been determined exactly [1,2,[8][9][10][11]. On the other hand, much less is known about its dynamics. This is in contrast to the PASEP on a ring for which exact results using Bethe's ansatz have been available for a long time [12,13]. For open boundaries there have been several studies of dynamical properties based mainly on numerical and phenomenological methods [14,15]. In this Letter we employ Bethe's ansatz to obtain exact results for the approach to stationarity at large times in the PASEP with open boundaries. Upon varying the boundary rates, we find crossovers in massive regions, with dynamic exponents z = 0, and between massive and scaling regions with diffusive (z = 2) and KPZ (z = 3/2) behaviour.The dynamical rules of the PASEP are as follows. At any given time t each site is either occupied by a particle or empty and the system evolves subject to the following rules. In the bulk of the system (i = 2, . . . , L − 1) a particle attempts to hop one site to the right with rate p and one site to the left with rate q. The hop is executed unless the neighbouring site is occupied, in which case nothing happens. On the first and last sites these rules are modified. If site i = 1 is empty, a particle may enter the system with rate α. If on the other hand site 1 is occupied by a particle, the latter will leave the system with rate γ. Similarly, at i = L particles are injected and extracted with rates δ and β respectively. With every site i we associate a Boolean variable τ i , indicating whether a particle is present (τ i = 1) or not (τ i = 0). The state of the system at time t is then characterized by the probability distribu...