This document reviews various methods for detecting counterfeit currency using image processing and machine learning techniques. It discusses seven different methods: 1) An AlexNet-based CNN system that achieves 81.5% accuracy for real notes and 75% for fake notes. 2) A deep CNN model with 85.6% accuracy. 3) A feature ensemble approach using classifiers like SVM, LDA, KNN that achieves 88.5% accuracy. 4) Four CNN architectures (AlexNet, Darknet-53, GoogleNet, ResNet-50) that achieve accuracies ranging from 64.64% to 80.94%. 5) Three machine learning algorithms (KNN, SVC, GBC) that achieve over 97% accuracy.