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Frequency-agile radio systems need to decide which frequencies are safe to use. In the context of recycling spectrum that may already be in use by primary users, both the spatial dimension to the spectrum sensing problem and the role of wireless fading are critical. It turns out that the traditional hypothesis testing framework for evaluating sensing misses both of these and thereby gives misleading intuitions. A unified framework is presented here in which the natural ROC curve correctly captures the two features desired from a spectrum sensing system: safety to primary users and performance for the secondary users. It is the trade-off between these two that is fundamental. The spectrum holes being sensed also span both time and space. The single-radio energy detector is used to illustrate the tension between the performance in time and the performance in space for a fixed value of protection to the primary user.