This document describes the ART1 neural network model, an unsupervised learning algorithm. ART1 performs clustering of binary input patterns. It has two layers of units (F1 and F2) with adaptive weights between them. The vigilance parameter ρ controls the level of similarity for patterns to be assigned to the same cluster. During training, an input is presented and the most active F2 unit is selected as the winner. Weights are updated only if the input and winner activation are sufficiently similar as determined by ρ. Otherwise, the network resets and searches for a better match. This continues until all patterns are clustered.