In recent years, the field of nanotechnology has witnessed remarkable advancement, leading to the emergence of an extensive “Nanoparticle Zoo” comprising diverse nanostructures such as nanotubes, nanospheres, nanostars, nanocapsules, nanodots, nanobubbles, nanorods, nanoshells, and nanoclusters, among others. Due to their high surface energy, nanoparticles tend to be unstable, displaying elevated reactivity and limited structural stability. This dual nature can have both positive and negative effects on nanoparticle processing. Current studies focus on high reactivity, neglecting poor stability. However, unstable processes can lead to unique nanomaterials, such as chemical transformation, assembly, or selective etching. This chapter delves into the stability and reactivity of nanoparticles, highlighting their fundamental concepts, distinct properties, and real-world uses. It covers dangling bonds, surface chemistry, dispersion, and thermal, structural, and chemical stability/reactivity. Slow progress in nano-based research attributed to the prime challenges are Specialized and expensive reagent usage, Inadequate procedures for handling nanomaterials can cause end users to manipulate the samples, large scale production are limited and lack of durability and repeatability in the surface functionalization of nanoparticles.