Determining the scaling relations between galaxy cluster observables requires large samples of uniformly observed clusters. We measure the mean X- ray luminosity-optical richness (image -image) relation for an approximately volume-limited sample of more than 17,000 optically selected clusters from the maxBCG catalog spanning the redshift range image. By stacking the X-ray emission from many clusters using ROSAT All-Sky Survey data, we are able to measure mean X-ray luminosities to image10% (including systematic errors) for clusters in nine independent optical richness bins. In addition, we are able to crudely measure individual X-ray emission from image800 of the richest clusters. Assuming a lognormal form for the scatter in the L sub(X)-image relation, we measure image at fixed image. This scatter is large enough to significantly bias the mean stacked relation. The corrected median relation can be parameterized by image, where image and image. We find that X-ray-selected clusters are significantly brighter than optically selected clusters at a given optical richness. This selection bias explains the apparently X-ray-underluminous nature of optically selected cluster catalogs.