Pervasive wireless computing and communication have created an ever-in-creasing demand for more radio spectrum. Since, most of the spectrum is underutilized, it motivated the introduction of the concept of cognitive radios, a dynamic spectrum ac-cess enabling technology. The first stage of cognitive radio is to sense the environment and determine which parts of the spectrum are available. This is achieved through spectrum sensing. However, spectrum sensing poses the most fundamental challenge in cognitive radios. Moreover, cognitive radios suffer from many vulnerabilities and the security attacks can severely degrade the performance of cog-nitive radios. This paper surveys state-of-the-art research on spectrum sensing and security threats in cognitive radios. Lastly, we also consider the analysis of issues related to spec-trum handoffs in cognitive radios.