This chapter presents the double-slit experiment and Davisson-Germer’s corresponding experiment that proved the wave-like nature of the electron in 1927. Next, it shows the basics of the special theory of relativity which is used by de Broglie to obtain the wave-particle duality. Afterwards, it displays the most important properties of a periodic wave along with the concepts of travelling wave, composite wave, group velocity, wave packet and single wave packet. These sections are the basis for understanding the de Broglie’s work where he used the special theory of relativity, Einstein’s quantum theory of light and the features of the periodic waves to obtain his famous constant relation of the product of linear momentum (of a particle) and wavelength (of a wave) which became the most important quantum concept used by Schrödinger to develop the quantum wave theory. In the end, a do-it-yourself activity is also given in order to plot several wave packets. (153 words).