Electromechanical oscillations are inherent to interconnected power systems. However, the frequency of the oscillations and the number of generators that oscillate in any electromechanical oscillatory mode depend on the structure of the power system network. Low-frequency electromechanical oscillations occur when existing generation/load areas are connected to other similar areas by relatively weak transmission lines. In this chapter, we will examine in more detail the relationship between low-frequency modes and weak interconnections in the transmission system network. The basis of the analysis will be the observation that generators in specific areas of a power system behave coherently in low-frequency electromechanical oscillations and that groups of coherent generators are separated from other groups of coherent generators by weak interconnections.