Conditions leading to plugging of fractures and slots by particles of different size, shape, composition and concentration are investigated experimentally. Empirical correlations are developed on the basis of relevant dimensionless numbers which determine the effect of aspect ratio (length-to-width ratio) of fracture opening and particle size distribution on plugging conditions. Plugging capabilities and fluid loss through fractures and slots are found to depend on the particle shape, size, composition, particle concentration, flow conditions, and fluid properties. It is determined that plugging occurs when the ratio of the fracture width to average particle diameter versus Particle-Volume-Fraction Reynolds number falls into the region below the curve described by a dimensionless empirical exponential correlation. Expressed in dimensionless numbers, such a relationship is directly applicable for estimation of whether plugging will occur and the time required for plugging under prescribed field conditions. The dimensionless correlations developed in this paper can also be used as a screening tool for best lost circulation material (LCM) candidates.