An important technique for determining an exoplanet’s mass is the measurement of the orbital semi-major axis. This comes from radial velocity (RV) measurements, together with estimates of stellar mass and the inclination of the orbit relative to the line of sight. If the exoplanet also transits its host star, then the resulting measurement of the exoplanet’s radius yields a value for density and likely chemical composition. Limitations on the accuracy and precision of RV measurements, both stellar and instrumental, degrade orbital determinations and thus the inferred properties of the exoplanet. There are at least four sources of RV jitter (also called stellar noise) from solar-like stars: granulation, supergranulation, magnetic effects on flow patterns, and pulsations. An additional source of RV jitter is instrumental including the limitations of data analysis software. This chapter explores each RV noise source and the limitations that the sum of these sources place on the ability to identify an exoplanet from radial velocity measurements. Several very different approaches have been developed to characterize and minimize the sources of RV noise.