Side-channel attacks are a serious threat to implementations of cryptographic algorithms. Secret information is recovered based on power consumption, electromagnetic emanations or any other form of physical information leakage. Template attacks are probabilistic side-channel attacks, which assume a Gaussian noise model. Using the maximum likelihood principle enables us to reveal (part of) the secret for each set of recordings (i.e., leakage trace). In practice, however, the major concerns are (i) how to select the points of interest of the traces, (ii) how to choose the minimal distance between these points, and (iii) how many points of interest are needed for attacking. So far, only heuristics were provided. In this work, we propose to perform template attacks in the principal subspace of the traces. This new type of attack addresses all practical issues in principled way and automatically. The approach is validated by attacking stream ciphers such as RC4. We also report analysis results of template style attacks against an FPGA implementation of AES Rijndael. Roughly, the template attack we carried out requires five time less encrypted messages than the best reported correlation attack against similar block cipher implementations.