This chapter provides a comprehensive examination of electromigration failures in advanced semiconductor devices. As demand for high-performance semiconductors grows, heterogeneous integration through 3D monolithic and 2.5D/3D advanced packaging technologies significantly boosts system performance but also dramatically increases current density. For instance, modern Nvidia GPU chips exceeding 1000 W create intense current densities that impact interconnect metal lines, TSVs, solder bumps, copper bumps, and RDLs across multiple interconnect levels, eventually merging into solder balls at the board level. Furthermore, with increased 2.5D/3D integration and chip stacking, thermomigration exacerbates interconnect reliability challenges due to the resulting temperature gradient. The chapter begins with a foundational understanding of electromigration mechanisms. Traditional design rules, based on Blech’s theory, are introduced and explained, including their use in introducing the concept of back stress, and threshold condition for the immunity from electromigration failure. This chapter focuses on new experimental data analysis and the development of advanced fully coupled models. Newly proposed design rules and acceleration models are presented, accounting for these critical factors and providing robust predictive tools for reliability assessment. The chapter also discusses the implementation of these advanced models in finite element analysis (FEA) using tools such as ANSYS and COMSOL.