The research objective of this paper is to develop an acoustic impedance model for micro-perforate plates. Passive acoustic liners consisting of perforate plates-over-honeycomb structures are a key contributor in the reduction of fan noise propagated through the inlet and aft-fan duct of aircraft engine nacelles. These perforated plates are physically characterized by plate thickness, t, orifice diameter, d, and porosity, σ, and the resultant liners are typically classified as conventional for t/d ≈ 1 or micro-perforate for t/d ≫1. Micro-perforate plates are becoming more popular because of their acoustic linearity, i.e., insensitivity to sound level. The goal of the current research is to develop models to better understand the acoustic behavior of liners constructed with micro-perforate facesheets. The first step is to model the flow resistivity of micro-perforate plates, computed as the ratio of the static pressure drop across the plate to the mean flow through the orifices. This result is then used to model the acoustic impedance, which is defined as the ratio of the acoustic pressure to the normal component of acoustic particle velocity at the liner surface. These tests are conducted in the NASA Langley Raylometer and Normal Incidence Tube with samples spanning a range of 4% ≤ σ ≤ 20% and 5.0 ≤ t/d ≤ 7.14.