This chapter examines requirements elicitation for complex systems from a theoretical and practical perspective. System stakeholders, requirements sources, and the quality of requirements are presented with respect to the process, including an investigation into the roles of requirements engineers during elicitation. The main focus of the chapter is a review of existing requirements elicitation techniques and a survey of current trends and challenges. It is concluded with some views on the future direction of requirements elicitation in terms of research, practice and education. It is the intention of the authors that readers of this chapter will be sufficiently informed on the concepts, techniques, trends, and challenges of requirements elicitation to then apply this knowledge to system development projects in both industrial and academic environments.