Research on social networks raises formidable ethical issues that often fall outside existing regulations and guidelines. Even standard informed consent and anonymization are difficult to implement with data about personal relationships. Today, state-of-the-art tools to collect, handle, and store personal data expose both researchers and participants to new risks. Political, military and corporate interests interfere with scientific priorities and practices, while legal and social ramifications of studies of personal ties and human networks come to the surface. The seven papers that form the special issue explore different aspects of ethical issues in contemporary social networks research. The special issue also includes a broad introduction by the guest editors and two invited comments.