When we deal with light in physics, we generally assume it to be a monochromatic wave. However, having a monochromatic wave is impossible; we often have quasi-monochromatic sources. This leads to partial coherence in light fields. Quantifying this coherence property of light and being able to measure it can tell us about the statistical nature of the source. This coherence measurement can be either first-order (correlation in electric field) or second-order (correlation in intensity). Each can tell us different properties about the source. In this talk, we will discuss the origin of coherence and ways to measure these properties in various systems.