Subsampling is an effective way to deal with big data problems and many subsampling approaches have been proposed for different models, such as leverage sampling for linear regression models and local case control sampling for logistic regression models. In this article, we focus on optimal subsampling methods, which draw samples according to optimal subsampling probabilities formulated by minimizing some function of the asymptotic distribution. The optimal subsampling methods have been investigated to include logistic regression models, softmax regression models, generalized linear models, quantile regression models, and quasi-likelihood estimation. Real data examples are provided to show how optimal subsampling methods are applied.