Energetic, highly collimated, episodic jets are observed in active galactic nuclei, binary stellar systems harbouring a black hole or white dwarf, and in association with stars in the process of formation. The common feature of these systems is that gas is fed onto the central object through an accretion disk. Time-dependent magnetohydrodynamical simulations show that magnetic fields anchored in the accretion disk can accelerate and collimate outflows into jets. For certain magnetic field configurations, episodic knots within the jet are produced.