Although visual attention has been subject to scientific inquiry for over a century, several basic questions are still unanswered. One key challenge to the study of covert spatial attention is that almost all experimental paradigms use, as their main measure, properties of the observer’s responses (e.g., accuracy or response time). However, responses can be influenced by many other factors that are often elusive to experimental control (e.g., higher-level strategies, experience with the task, response biases, and so on). In this talk I will describe studies that explored the allocation of covert attention with a measurement that is independent of performance – the pupillary light response (PLR; changes in pupil size that are related to luminance levels), thereby avoiding various obstacles and biases involved in more traditional measurements of covert attention. Specifically, I will describe studies that used attentional modulations of the PLR to test theories of object-based attention, and if time permits I will also discuss studies that used the PLR to examine the effects of perceptual load (or task difficulty) on attention allocation