In our modern information age, recent technical developments and trends, such as mobile and pervasive computing, big data, cloud computing, and Web 2.0 applications, increasingly pose privacy dilemmas. Due to the low costs and technical advances of storage technologies, masses of personal data can easily be stored. Once disclosed, these data may be retained forever, often without the knowledge of the individuals concerned, and be removed with difficulty. Hence, it has become hard for individuals to manage and control their personal spheres. Both legal and technical means are needed to protect privacy and to (re-)establish the individuals' control. This chapter provides an overview to the area of Privacy-Enhancing Technologies (PETs), which help to protect privacy by technically enforcing legal privacy principles. It will start with defining the legal foundations of PETs, and will present a classification of PETs as well as a definition of traditional privacy properties that PETs are addressing and metrics for measuring the level of privacy that PETs are providing. Then, a selection of the most relevant PETs is presented.