This document analyzes spectrum sensing using an energy detection technique in cognitive radio. It evaluates the performance of energy detection at signal-to-noise ratios (SNRs) of -10dB, -15dB, and -20dB. The energy detector is simple to implement and requires no knowledge of the transmitted signal. Simulation results show that the probability of detection increases with higher SNR values. Energy detection performance depends on the predefined probability of false alarm and detection in both additive white Gaussian noise and Rayleigh fading channels.