The role of particle counting instrumentation is examined for use in the municipal drinking water facility. The comparative construction of the particle counter and turbidimeter are described briefly, showing their differences in design and application. Particle count results from both pilot and full scale filters from various sites across the United States are presented and discussed, including particle count comparison to turbidity and optimization by choice of coagulant, polymer dosage, and flow rate. Membrane effluent monitoring comparisons using particle counts, turbidity, and cyst seeding results are also presented.