In this chapter, the discussion of oscillations is extended to systems with two and more degrees of freedom. This extension naturally leads to another key notion of physics - the waves, in particular those in uniform $$1 D$$ systems. (A more general discussion of elastic waves in continua is deferred until the next chapter.) However, even the limited scope of the models analyzed in this chapter will still enable us to discuss such important general aspects of waves as their dispersion, phase and group velocities, impedance, reflection, and attenuation.