In recent years speckle in optical projection microlithography received increasing interest because of its potential contribution to linewidth roughness (LWR). Speckle is a light interference effect that causes the dose delivered to the reticle to be nonuniform, causing a linewidth variation of the patterns imaged in the resist. The contrast of the speckle pattern is shown to be caused by a combination of temporal and spatial coherence effects of the light. The temporal part, determined by the bandwidth of the laser light and the duration of the laser pulse, is found to be the dominant contributor to speckle in today’s ArF optical lithography. The spatial distribution of the speckle pattern depends on the intensity distribution of the light in the pupil. Consequently, the spatial frequencies of the LWR induced by speckle depend on the illumination condition, which is confirmed experimentally by exposing wafers with different amounts of speckle contrast. The experiments demonstrate that the amplitude of the LWR induced by speckle is consistent with theory and simulations. Its amplitude is small compared to other sources of LWR, but it is clearly present and should not be ignored when extending ArF optical lithography into future technology nodes.