Solid lipid nanoparticles (SLN) are nanocarriers in the 10–1000 nm range of a solid core, containing both hydrophilic and hydrophobic active pharmaceutical ingredients. SLNs are composed of well-tolerated and biodegradable solid lipids such as mono-, di-, and triglycerides, fatty acids, waxes, and steroids, as well as lipophilic and hydrophilic emulsifying agents. This composition of biocompatible molecules makes SLNs one of the most successful options for the administration of drugs with different routes of administration. To determine its size, morphology, and surface charge, laser diffraction spectroscopy techniques, dynamic light scattering, coulter counter, scanning ion occlusion sensing, and advanced microscopy techniques such as scanning electron microscopy, transmission electron microscopy, and atomic force microscopy are some of the most widely used methods. Surface morphology and length can be measured by electron microscopy, while dynamic light scattering and photon correlation spectroscopy determine particle size and size distribution. In addition, colloidal stability can be determined by zeta potential analysis, indirect measurement of surface charge, and differential scanning calorimetry to characterize particles and drug interactions.