While the real world is the time domain, sometimes we can gain insight about the origin of problems by looking in the frequency domain. The time domain and frequency domain properties of power rail noise are a little different than for digital signals. For some types of components, such as analog parts, mixed signal or clock generation, very low levels of power rail noise can have a big impact on performance. Sometimes the best way of finding these noise sources is by frequency domain analysis. We will discuss best measurement practices using real-time oscilloscopes to analyze power rail noise in the time and frequency domains to provide insight into the root cause of the problems.