In this chapter, you have learned the basic concepts behind multilayer artificial neural networks, which are currently the hottest topics in machine learning research. In Chapter 2, Training Simple Machine Learning Algorithms for Classification, we started our journey with simple single-layer neural network structures and now we have connected multiple neurons to a powerful neural network architecture to solve complex problems such as handwritten digit recognition. We demystified the popular backpropagation algorithm, which is one of the building blocks of many neural network models that are used in deep learning. After learning about the backpropagation algorithm in this chapter, we are well-equipped for exploring more complex deep neural network architectures. In the remaining chapters, we will introduce TensorFlow, an open source library geared towards deep learning, which allows us to implement and train multilayer neural networks more efficiently.