A layer is container of neurons.  Layers: - Input - Hidden (zero or more) - Output Neurons are placed within layers and each layer has a purpose > The neurons, within each of the layer of a neural network, perform the same function. They simply calculate the weighted sum of inputs and weights, add the bias and execute an activation function. ### Input Layer Inputs can be loaded from external source such as web service or csv file  ### Output Layer  ### Hidden Layer Usually each hidden layer contains the same number of neurons The larger the number of hidden layers in a neural network, the longer it will take for the neural network to produce the output and the more complex problems the neural network can solve.