Due to its versatility, flexibility and fast development, the modern Internet is far from being well understood in its entirety. A good way to learn more about how the Internet functions is to collect and analyze real Internet traffic. This paper addresses several major challenges of Internet traffic monitoring, which is a prerequisite for performing traffic analysis. The issues discussed will eventually appear when planning to conduct passive measurements on high-speed network connections, such as Internet backbone links. After giving a brief summary of general network measurement approaches, a detailed overview of different design options and important considerations for backbone measurements is given. The challenges are discussed in order of their chronological appearance: First, a number of legal and ethical issues have to be sorted out with legislators and network operators, followed by operational difficulties that need to be solved. Once these legal and operational obstacles ha...