This article begins with a description of space- and object-based guidance of attentional selection. It goes on to discuss the most influential, two-rectangle, paradigm for demonstrating the existence of space- and object-based attentional effects. The article then considers two different mechanisms, attentional spreading and attentional prioritization, that can potentially explain how object representations come to guide attentional selection. Finally, it discusses several empirical findings that have emerged in support of the two different mechanisms. It concludes by putting forth a new framework for investigating object-based effects. WIREs Cogn Sci 2012, 3:163-169. doi: 10.1002/wcs.1162 For further resources related to this article, please visit the WIREs website.