On dynamical adjustment mechanisms for the cosmological constant

After recalling why dynamical adjustment mechanisms represent a particularly attractive possibility for solving the cosmological constant problem, we briefly discuss their intrinsic difficulties as summarized in Weinberg's no-go theorem. We then comment on some problems of the recently proposed 'self-tuning' mechanism in 4+1 dimensions. Finally, we describe an alternative approach which uses the time-evolution of the universe to achieve a dynamical relaxation of the cosmological constant to zero.