The objective to induce reproducible, efficient, and stable second-order nonlinear optical properties (SONL) in an isotropic material remains a challenge in photonics. Thermal poling allows inducing SONL properties in glasses by the formation of an axial symmetry because of the implementation of a static electric field within the glassy matrix. A description of the main poling mechanisms is proposed. Depending on the glass compositions and the poling conditions, an effort is made to correlate poling mechanisms and the strength of the implemented static electric field. A review of the main technological improvements done to develop poled-silica-based electro-optical modulators as well as the SONL efficiencies obtained for different glass compositions is reported and analyzed.