Many tools are available for water treatment plant operators and engineers to use in evaluating filter performance. Particle counting is one technology being implemented in many treatment plants to augment filter effluent monitoring. When implemented correctly, particle counting can be a valuable tool in gaining additional information and insight into filter operation and performance that may not be evident through the use of traditional turbidimeters. Information gained from using particle counting technology may assist in identifying early breakthrough of filters, optimizing filter backwash procedures and filter to waste operations, and assist in overall plant operations. As many plants begin to proceed with biologically active carbon filters (BAC) and as regulations become more stringent, a deeper understanding of how individual filters perform is crucial in maintaining compliance and ensuring the protection of public health. Includes 7 references, figures.