The objective of this chapter is to show how it would be possible to introduce a certain degree of tunability of the properties of phononic crystals. The main concepts underlying the conception of tunable phononic crystals are first introduced with simple models: the one-dimensional harmonic crystal with varying parameter and two coupled one-dimensional harmonic crystals. An overview of the literature on tunable phononic crystals is given. Three of the tuning methods proposed in the literature are described in some details. We also illustrate the new or enhanced functionalities open by the tuning of the phononic crystal properties. These applications include reconfigurable waveguides and tunable superlenses.