A lithographic apparatus includes an illuminator for receiving a beam of EUV radiation from a radiation source apparatus and for conditioning the beam to illuminate a target area of a patterning device, such as a reticle. The reticle forms a patterned radiation beam. A projection system transfers the pattern from said patterning device to a substrate by EUV lithography. Sensors are provided for detecting a residual asymmetry in the conditioned beam as the beam approaches the reticle, particularly in a non-scanning direction. A feedback control signal is generated to adjust a parameter of said radiation source in response to detected asymmetry. The feedback is based on a ratio of intensities measured by two sensors at opposite ends of an illumination slit, and adjusts the timing of laser pulses generating an EUV-emitting plasma.