The fascinating relationship between structure and property in nanowires has enabled a wealth of applications in photonics and electronics. The behavior of phonons in nanowires is also modified compared to their bulk counterparts. In this review, we provide an overview of the recent efforts to investigate the properties of acoustic phonons in nanowires using ultrafast optical methods. In particular, we focus on the calculation of the modified phonon dispersion in nanowires and how to address them optically. We then discuss experimental investigations in arrays of nanowires and a single nanowire. The analysis of phonon behavior reveals the possibility to perform advanced mechanical characterization and to vary the thermal properties of nanowires. The review concludes with a brief perspective on future research directions, from phonon-induced control over properties to three-dimensional (3D) acoustic nano-imaging.