Training a target neural network comprises providing a first batch of samples of a given class to respective instances of a generative neural network, each instance providing a variant of the sample in accordance with the parameters of the generative network. Each variant produced by the generative network is compared with another sample of the class to provide a first loss function for the generative network. A second batch of samples is provided to the target neural network, at least some of the samples comprising variants produced by the generative network. A second loss function is determined for the target neural network by comparing outputs of instances of the target neural network to one or more targets for the neural network. The parameters for the target neural network are updated using the second loss function and the parameters for the generative network are updated using the first and second loss functions.