In recent years, there have been an increase in recorded oscillations that cannot be simply explained by examining the model of the system. These sustained oscillations are initiated by an external or unintended periodic input to the power system and are called forced oscillations. The increased number of recorded forced oscillation events can be analyzed using wide-area measurement systems based on phasor measurement units, which provide improved grid observability. A common cause of forced oscillations is malfunction of a steam valve at a thermal power plant such that the valve cycles between the on and off positions due to either a mechanical or control system failure. This kind of malfunction leads to four major forced-oscillation events in the North America power grids. In order to quickly mitigate negative consequences of a forced oscillation, it is necessary to identify the source of the oscillation and either correct the malfunctioning source or disconnect it from the grid. This chapter describes factors influencing oscillation source identification and practical approaches to locate the source of the oscillation. The techniques are demonstrated in a set of examples using power system models of different complexity as well as actual measurements from the North America Eastern Interconnection.