Stability of a dosage form is its ability to preserve its quality attributes within preset limits. The time span over which these attributes remain within specifications is the shelf-life of the drug product. Stability is a very complex feature and is influenced not only by the stability of the drug substance but also by the stability of excipients and the interaction of the components within the system. Another important contributing factor is the packaging material, which is responsible for the protection of the drug product. Not only drug substances, but also excipients are susceptible to different degradation mechanisms. Amorphous polymers, a relatively frequently used group of excipients, are especially prone to physical instability. Through the process of physical ageing, a slow volume and enthalpy relaxation can be experienced, which can lead to remarkable alterations in solid dosage form properties. Functional changes within the solid dosage form associated with instability include changes in mechanical properties, homogeneity and drug release characteristics, discoloration, phase separation or changes in melting time of suppositories. Stability assessment is a crucial issue during formulation development, which is strictly regulated by authorities responsible for drug registration. The primary purpose of this paper is to give an overview of the different types of physical changes influencing solid state stability of dosage forms, and how such changes can be monitored. We will also illustrate how the quality of a solid dosage form varies with time under the influence of different environmental factors.