Hydrophilic interaction liquid chromatography (HILIC) is a popular technique for the separation of polar compounds, which are poorly retained by reversed-phase liquid chromatography. Despite the versatility and the potentiality of this technique, many analysts still feel uncomfortable when approaching it. The HILIC retention mechanism is not completely elucidated and the availability of many different stationary phases may be confusing during method development. Understanding the principles that drive the separation and how they can be influenced by the selection of both stationary phase and chromatographic conditions enhances the range and the quality of possible applications. For this purpose, the review discusses the primary interactions at the basis of HILIC separations and presents an overview of the most common HILIC stationary phases. The effects of the stationary phase type and chromatographic parameters (i.e., organic solvent and salt content, mobile phase pH and column temperature) on each specific interaction are highlighted.