In this chapter we are particularly interested in requirements engineering of software where the software is part of a complex engineered system; that is, embedded software. Embedded software is the software that runs on a computer system that is integral to a larger system whose primary purpose is not computational. Embedded software systems are of rising significance in the industry, and they are found in a wide range of industries in the modern world, including medical, nuclear, chemical, rail networks, aerospace, and automotive industries. The RE of this category of software is challenging because of its special properties that add to its complexity and make it more expensive and error-prone as compared with other software categories, for example, business applications. In this chapter we identify the special properties of embedded software systems to help in better understanding of such domain, discuss the special RE challenges that the special properties introduce, and introduce the main current RE approaches for the domain.