The chapter discusses the stability and size distribution of soil aggregates, which are groups of primary particles that cohesionally adhere to each other more strongly than to surrounding soil particles. The stability of aggregates is influenced by the cohesive forces between particles, which must withstand disruptive forces such as impact, shearing, and water entry. Standardizing these disruptive forces is crucial for comparing aggregate size distributions and stability in different contexts. For dry aggregates, the primary forces involved include surface tension and cohesive tension, which pull particles together as the soil dries. Wind erosion is a significant natural force that disintegrates dry soil, and methods like the rotary sieve are used to measure aggregate size distribution and stability. The rotary sieve is effective but can be expensive and requires specialized equipment. For wet aggregates, the primary factor affecting stability is the method of wetting. Slow wetting under atmospheric pressure or vacuum results in less disintegration compared to rapid wetting at atmospheric pressure. Techniques like elutriation and sedimentation are used to determine aggregate size distribution, but they have limitations, such as the rapid settling of larger aggregates. The chapter also discusses methods for expressing aggregate size distribution, including mean weight diameter (MWD) and geometric mean diameter (GMD). MWD is calculated by summing the products of the mean diameter of each size fraction and the proportion of the total sample weight in that fraction. GMD is calculated using logarithmic transformations and is useful for soils with log-normal size distributions. Finally, the chapter provides detailed procedures for determining aggregate stability and size distribution, emphasizing the importance of standardized methods and the impact of various factors such as water content, temperature, and soil texture on the results.The chapter discusses the stability and size distribution of soil aggregates, which are groups of primary particles that cohesionally adhere to each other more strongly than to surrounding soil particles. The stability of aggregates is influenced by the cohesive forces between particles, which must withstand disruptive forces such as impact, shearing, and water entry. Standardizing these disruptive forces is crucial for comparing aggregate size distributions and stability in different contexts. For dry aggregates, the primary forces involved include surface tension and cohesive tension, which pull particles together as the soil dries. Wind erosion is a significant natural force that disintegrates dry soil, and methods like the rotary sieve are used to measure aggregate size distribution and stability. The rotary sieve is effective but can be expensive and requires specialized equipment. For wet aggregates, the primary factor affecting stability is the method of wetting. Slow wetting under atmospheric pressure or vacuum results in less disintegration compared to rapid wetting at atmospheric pressure. Techniques like elutriation and sedimentation are used to determine aggregate size distribution, but they have limitations, such as the rapid settling of larger aggregates. The chapter also discusses methods for expressing aggregate size distribution, including mean weight diameter (MWD) and geometric mean diameter (GMD). MWD is calculated by summing the products of the mean diameter of each size fraction and the proportion of the total sample weight in that fraction. GMD is calculated using logarithmic transformations and is useful for soils with log-normal size distributions. Finally, the chapter provides detailed procedures for determining aggregate stability and size distribution, emphasizing the importance of standardized methods and the impact of various factors such as water content, temperature, and soil texture on the results.