Over the past few years, the PAC-Bayesian approach has been applied to numerous settings, including classification, high-dimensional sparse regression, image denoising and reconstruction of large random matrices, recommendation systems and collaborative filtering, binary ranking, online ranking, transfer learning, multiview learning, signal processing, to name but a few. The "PAC-Bayes" query on arXiv illustrates how PAC-Bayes is quickly re-emerging as a principled theory to efficiently address modern machine learning topics, such as leaning with heavy-tailed and dependent data, or deep neural networks generalisation abilities. This tutorial aims at providing the ICML audience with a comprehensive overview of PAC-Bayes, starting from statistical learning theory (complexity terms analysis, generalisation and oracle bounds) and covering algorithmic (actual implementation of PAC-Bayesian algorithms) developments, up to the most recent PAC-Bayesian analyses of deep neural networks generalisation abilities. We intend to address the largest audience, with an elementary background in probability theory and statistical learning, although all key concepts will be covered from scratch.