The recent advent of the phasor measurement unit (PMU) and their associated networks has revealed that forced oscillations are a common occurrence in synchronized power systems. Forced oscillations are not be confused with electromechanical oscillations. Forced oscillations are typically caused by some rouge input driving the system into a sustained oscillation. An example is an ill-operating power-plant steam valve oscillating in a limit-cycle. While the existence of forced oscillations has been recognized for many decades, their dynamical impact on the system is not well established. This paper expands on the current knowledge by proposing a dynamic model for forced oscillations and uses this model to explore how the forced oscillations are perceived throughout the system. Specifically, we introduce the concept of “forced-oscillation shape” and examine how this shape interacts with the modal dynamics of a power system. We use simulation examples to demonstrate key points.