Nanosuspensions are nanosized colloidal dispersion systems that are stabilized by surfactants and/or polymers. Because nanosizing results in the creation of new interfaces and in a positive Gibbs free energy change, nanosuspensions are thermodynamically unstable systems with a tendency toward agglomeration or crystal growth. Despite extensive research on nanosuspension technology, stability remains a limitation for pharmaceutical or industrial applications of nanosuspensions. Furthermore, the empirical relationship between stabilizer efficacy and nanosuspension stability has not been well characterized. This review focuses on the issue of nanosuspension stability in drug delivery to present the state of the art of nanosuspensions. Therefore, this review will discuss unstable suspensions, methods and guidelines for selecting and optimizing stabilizers, approaches for enhancing stability, and other factors that influence nanosuspension stability. This review could serve as a reference for the educated selection of a stabilizer for a specific drug candidate and the optimization of the operational parameters for nanosuspension formulation, rather than the currently practiced trial-and-error approach.