Advancement in nanotechnology has led to its application in a plethora of domains, i.e., catalysis,^1,2  water treatment,^3,4  electronics,^5,6  composite materials,^7,8  CO₂ sequestration,^9,10  thermal engineering,^11–15  lubricants,^16–18  medicine,^19–21  medical diagnostics,^22–24  supercapacitors,^25,26  and so on. The utilization of colloidal suspensions of nanoparticles (nanofluids) has gained popularity in several domains, as coolant,^27–31  nano lubricants,^32–34  medical diagnostics,^35–37  use in CO₂ sequestration,^38–40  and so on. The invention of nanofluids was inspired by the limitations of micron-sized particle suspensions, which have the following shortcomings: poor suspension stability, particle deposition tendency, pipeline erosion, higher particle loading requirements, augmented viscosity, and increased pumping costs, which prevent their widespread industrial application despite their moderate thermal conductivity.^13,41  With the development of nanofluids in the Argonne National Laboratory back in the 1990s by Stefan Choi,⁴²  most of the aforementioned problems were eliminated. The beneficial attributes of nanofluids, such as high thermal conductivity, low viscosity increment, low particle loading, altered specific heat, and ease of application in micro channels, have increased their usability in various domains.^{30,41,43–51}  However, issues with the stability of nanofluids largely prevent industry from utilizing them.^52–56  The current chapter focuses specifically on the stability aspects of nanofluids to explore the underlying mechanism and identify the major factors responsible for their destabilization. The recognition of key factors related to the stability of nanofluids, such as the role of nanoparticles, base fluids, synthesis techniques, operating conditions, external forces, and stabilization techniques, is essential for maintaining their long-term stability. In addition to the above factors, compatibility between base fluid–nanoparticle–stabilizers and the applicational suitability of these three components are crucial for stabilization purposes. This chapter provides a detailed insight into Derjaguin-Landau-Verwey-Overbeek (DLVO)^57,58  and non-DLVO^59,60  theory and how they play a critical role in colloidal destabilization. As the chapter primarily focuses on aspects of the stability of nanofluids, stabilization techniques are the center of attention. The stabilization techniques covered in this chapter include a discussion on both mechanical and chemical stabilization.^52,55,61  Mechanical stabilization can be achieved using ultrasonic, magnetic stirring, vortex shaking, and ball milling techniques, whereas chemical stabilization can be broadly classified into three distinctive classifications, i.e., electrostatic, steric, and electrosteric stabilization. All of these techniques are discussed in detail in the chapter. However, all of these features have also been discussed extensively in the literature. This chapter also summarizes the operating principles of different stability evaluation methods, such as zeta potential analysis, dynamic light scattering (DLS), centrifugation, UV–visible spectroscopy, 3ω methods, etc.^52,55,62  To provide a clear understanding for readers on what the ideal properties of nanofluids are and the guidelines necessary for maintaining their long-term stability, parameters such as particle size–shape, zeta potential, nanoparticle–base fluid compatibility, thermal and oxidation stability of the nanoparticles and base fluid, the density difference between the base fluid and nanoparticles, high dielectric constant, boiling and freezing points of the base fluid, and viscosity of the base fluid are also discussed. It should be pointed out that discussion on the challenges of long-term nanofluid stabilization remains relatively unexplored, which is extensively addressed in this chapter. The long-term stability of nanofluids is highly important to ensure their reusability in industrial, environmental, and medical applications. Instability in colloidal suspension makes them unsuitable for any applications. Therefore, discussion on this topic is necessary to offer researchers and industry professionals, adequate ideas to assess and mitigate the complications in attaining long-term nanofluid stability. Guidelines have been chalked out to provide a possible roadmap to achieve the long-term stability of nanofluids. Stabilizer addition is considered to be one such route to this goal. However, several limitations exist in this approach as the addition of additives may also lead to a decline in several important parameters such as thermophysical characteristics (specific heat, thermal conductivity, and viscosity) and give rise to other problems such as foaming tendency, thermal degradation, poor freezing point and even lead to destabilization in a saline environment. Therefore, to ensure maximum benefits of stabilizer addition, the correct type of stabilizer needs to be selected based on nanoparticle type, operating conditions, and base fluid properties, which is also discussed herein. This chapter concludes with a discussion on future research directions.