In this context, Industry 4.0 has been developed in different ways and by different agents (Research Centres, Universities, Large Companies, Technological SMEs...), encompassing many elds and applications. Among all the involved technologies, some of the most relevant are: IoT and Cybernetic Systems, Additive Manufacturing & 3D Printing; Big Data, Data Mining & Data Analytics; Arti cial Intelligence, Collaborative Robotics (Cobot), Virtual Reality and Augmented Reality, among others. Currently, there seems to be a full consensus among experts about the economic impact that this revolution will bring. However, as a conse- quence of this scenario, the World Economic Forum (WEF) [1] estimates a destruction of approximately 5 million jobs, with the consequent socio-economic impact on the different countries. On the other hand, other experts state that the incoming changes will result on the crea- tion of new job requirements that have not yet been created and where more than half of current high degree students will work. Therefore, it will be necessary to wait to know the de nite scenario, since experts like Robert Tornabell Carrio [2], believe that the job reduction because Industry 4.0 can be balanced by the creation of new jobs. Consequently, this fourth industrial revolution is becoming an open and variable de nition, where depending on the conditions and capa- bilities of each company can lead to different solutions. Nevertheless, Industry 4.0 can be summarized as the introduction of the connectivity into the factories, in order to store and process information. Obviously, before reaching this state, it is necessary to introduce full automation systems. In this line, the next sections will analyse the main technolo- gies applied to manufacturing such as augmented reality, simulation and virtualization tools, process monitoring or additive manufacturing, highlighting their relevance and interest in order to bring the factory closer to the concept of Industry 4.0. Augmented reality Augmented reality (RA) or virtual reality (VR)? These two terms have been introduced within this fourth industrial revolution. It is easy to use them indistinctly, since both are referred to link the real and the virtual world. Nevertheless, they present different connotations. While virtual reality objective is to immerse the user in the virtual world, augmented reality lets the user feel the real environment, overlapping virtual objects in the real environment. In the following scheme gure 2 shows the trend of these two terms to the real and virtual world respectively. There are different de nitions of augmented reality. One of them is published in the Handbook of Augmented Reality and de ne de Augmented Reality as the direct or indirect real-time view of a real physical environment that has been augmented or improved, adding virtual information. One of the main characteristics of Augmented Reality is that needs to be interactive with the external environment and dynamic, combining real and virtual objects in 3D. The main objective of Augmented Reality is to improve, simplify and/ or facilitates the user's tasks, since it increases both their perception and interaction with the real world. Main applications in manufactu- ring are the training of operators for complex assemblies (projecting the instructions on glasses or screens) or the remote maintenance, where virtual instructions can be sent from the company headquarters to the operator. Figure 1. Main milestones for the Industry 4.0 implementation. Augmented Reality • Overlaps virtual elements in the real enviroment • Main industrial applications. Mantainance, training, modi cations,.... Virtual Reality • Createa full virtual environment • Main industrial applications: Simulators, training, commercial, ... Figure 2. Difference between Augmented Reality and Virtual Reality (Examples by Tecnalia). Referring to the industrial implementation of AR, three types of technologies can be differentiated to implement augmented reality: Head-deployed devices (HMDs), handheld devices and space devices (SARs). An example of each is shown in next gure. It is very common to join AR with smart-glasses. However, the current trend is the use of smartphones and tablets in AR applications. These devices are so common, cheap and accessible that it is dif cult to think of a trend change. Companies like iAR (Industrial Augmented Reality), defends the idea of eliminating the static screens that today each machine disposes and replace it with a smartphone or tablet screen, valid for the different machines of a plant. INDUSTRY 4.0 >>49