**1. Introduction**

The search for new semiconductor materials began with new technology requirements in the early nineteenth century [1]. One of the pivotal discoveries was silicon (Si) and germanium (Ge), by Clemens Winkler in 1886. The current semiconductor technology is mostly based on Si material to fabricate integrated circuits (ICs) in the era of high-speed Internet-of-Things (IoT). Since Si transistors of ICs have faced physical limitations due to their fundamental properties, a number of researchers are actively searching for alternatives, which reignite the active study of Ge to break through the technology roadblock. The scope of this introduction is to describe the historical background of Ge materials from ores and their application to advanced devices such as photodetectors, solar cells, spintronics, IC, etc., which are essentially semiconductor applications in all areas of our current technologies [2]. Furthermore, this chapter will discuss the opportunities and challenges of Ge materials and advanced device applications for the next technology generation. The last part of this section will outline the topic of each chapter with some practical suggestions on how to efficiently utilize this book for readers.
