8.EFTx is an online version of MIT's graduate Effective Field Theory course. The course follows the MIT on-campus class 8.851 as it was given by Professor Iain Stewart in the Fall of 2013, and includes his video lectures, resource material on various effective theories, and a series of problems to facilitate learning the material. Effective field theory (EFT) provides a fundamental framework to describe physical systems with quantum field theory. In this course you will learn both how to construct EFTs and how to apply them in a variety of situations. We will cover the majority of the common tools that are used by different effective field theories. In particular: identifying degrees of freedom and symmetries, formulating power counting expansions (both dimensional and non-dimensional), field redefinitions, bottom-up and top-down effective theories, fine-tuned effective theories, matching and Wilson coefficients, reparameterization invariance, and various examples of advanced renormalization group techniques. Examples of effective theories we will cover are the Standard Model as an effective field theory, integrating out the massive W, Z, Higgs, and top, chiral perturbation theory, non-relativistic effective field theories including those with a large scattering length, static sources and Heavy Quark Effective Theory (HQET), and a theory for collider physics, the Soft-Collinear Effective Theory (SCET). Course Flow Since this is an advanced graduate physics course, you will find that self motivation and interaction with others is essential to learning the material. The purpose of the online course is to set you up with a foundation, to teach you to speak the language of EFT, and to connect you with other students and researchers that are interested in learning or broadening their exposure to this subject. Each week you will complete automatically graded homework problems to test your understanding and to help you master the material. You are expected to discuss the homework with other people in the class, but your online responses must be done individually. To facilitate these interactions there will be a forum for student-to-student discussions, with threads to cover different topics, and moderators with experience in this field. Student learning and discussions will also be prompted by questions posed after each lecture topic. There will be no tests or final exam, but at the end of the course each student will give a 30 minute presentation on an EFT topic of their choosing. The subject of effective field theory is rich and diverse, and far broader than we will be able to cover in a single course. The presentations will create an opportunity for you to learn about additional subjects beyond those in lecture from your fellow students. To facilitate this learning opportunity, each student will be required to watch and grade eight presentations from among their fellow students. Of these eight, you will view 5 presentations that will be chosen for you, and you will view at least 3 more that you choose yourself from among the available topics.