Self-admitted technical debt (SATD) consists of annotations, left by developers as comments in the source code or elsewhere, as a reminder about pieces of software manifesting technical debt (TD), i.e., “not being ready yet”. While previous studies have investigated SATD management and its relationship with software quality, there is little understanding of the extent and circumstances to which developers admit TD. This paper reports the results of a study in which we asked developers from industry and open-source about their practices in annotating source code and other artifacts for self-admitting TD. The study consists of two phases. First, we conducted 10 interviews to gather a first understanding of the phenomenon and to prepare a survey questionnaire. Then, we surveyed 52 industrial developers as well as 49 contributors to open-source projects. Results of the study show how the TD annotation practices, as well as the typical content of SATD comments, are very similar between open-source and industry. At the same time, our results highlight how, while open-source code is spread of comments admitting the need for improvements, SATD in industry may be dictated by organizational guidelines but, at the same time, implicitly discouraged by the fear of admitting responsibilities. Results also highlight the need for tools helping developers to achieve a better TD awareness.