## 1 Introduction  
The current situation of assembly operations is characterized by an increasingly varied demand (mass customization) while the production faces a trade-off between the superior productivity of automated assembly systems and the absolute flexibility and adaptability of manual assembly. Therefore, high-volume production of discrete goods received heavy investments for automation, while low volume, made-to-order or engineer-to-order products were typically assembled manually [1; 2]. In this context, Lean production (a generalization of the Toyota Production System) expanded from its origin--automotive--to many other sectors and was adapted as necessary to the particularities of each industry or company [3]. Lean production typically focuses on value as perceived from the customer's point of view. Thus it considers that the flexibility to quickly adapt to market demand is critical. For Lean, rigid automation can be seen as a hindrance rather than an advantage, and seeks to incorporate the human factor to automation: _jidoka_, or \"automation with a human touch\" [4]. The term Industry 4.0, initially adopted by a German strategic program [5], is used nowadays to express the relationship between different elements of the current manufacturing sector and the new digital technologies. These Key Enabling Technologies are, according to [6]: Big data and analytics, Autonomous robots, Simulation, Horizontal and vertical system integration, the Industrial Internet of Things (IoT), Cybersecurity, The cloud, Additive manufacturing, and Augmented Reality. Recent research on Industry 4.0 tends to focus on the possibilities brought by a certain new digital technology or develops a framework to understand what would be the effect of incorporating such new technologies [7]. The arrival of the new digital technologies could address the aforementioned dichotomy of highly productive yet rigid automation vs. flexible but less-productive manual assembly. The quickly developing fields of human-robot collaboration, virtual/augmented reality and Automated quality control, to cite some examples, show promise in bringing forward actually flexible and adaptable automation that has the best of both worlds. Scarcely explored is the development of implementation methodologies that bridge Industry 4.0 conceptual frameworks with the current state of industrial environments and the process to successfully deploy new digital technologies that bring the expected returns of investment. Additionally, if the Lean production approach and its techniques are also related to this implementation, the concept of Lean 4.0 could be used, as shown in the literature [8]. Since Lean production and Industry 4.0 certainly have some commonalities [9], Lean could prove useful in providing a starting point for the implementation of Industry 4.0 technologies that improve assembly operations in a mass customization demand context. In order to assess the impact of any changes, careful evaluation systems are needed to ensure that technology investments are implemented to solve the problems and address business goals, and not just because they are available or they bring some cosmetic advantage. The 4 th Industrial Revolution is expected to transform the role of the people, but to what extent will assembly operators be affected--are humans to be replaced by machines or empowered by new technology? The issue that this literature review aims to address is: _How could Industry 4.0 technologies improve the flexibility, productivity and quality of assembly operations?_ To look into it, we aim to answer the following questions:  
1. What are the characteristics and implications of mass customization for assembly operations? 2. What new Industry 4.0 digital technologies are relevant to assembly operations?How to make the most out of their potential, and how to measure the improvement? 3. Is Lean production the best starting ground for implementing Industry 4.0 assembly operations? 4. How would Industry 4.0 affect people in assembly?