Technological systems are included as a component of national technology curricula and standards for primary and secondary education as well as corresponding teacher education around the world. Little is known, however, of how pupils, students, and teachers conceive of technological systems. In this article we report on a study investigating Swedish technology student teachers' conceptions of technological systems. The following research question is posed: How do Swedish technology student teachers conceive of technological systems? Data was collected through in-depth qualitative surveys with 26 Swedish technology student teachers. The data was analysed using a hermeneutic method, aided by a theoretical synthesis of established system theories ("system significants"). The main results of the study are that the technology student teachers expressed diverse conceptions of technological systems, but that on average almost half of them provided answers that were considered as undefined. The parts of the systems that the students understood were mostly the visible parts, either components, devices, or products such as buttons, power lines, hydroelectric plants, or the interface with the software inside a mobile phone. However, the "invisible" or abstract aspects of the technological systems, such as flows of information, energy or matter, or control operations were difficult to understand for the majority of the students. The flow of information was particularly challenging in this regard. The students could identify the input and often the output of the systems, that is, what systems or components "do," but the processes that take place within the systems were elusive. Comparing between technological systems also proved difficult for many students. The role of humans was considered important but it was mostly humans as users not as actors on a more systemic level, for example, as system owners, innovators, or politicians. This study confirms previous research in that the students had a basic understanding of structure, input and output of a technological system. Thus, the adult students in this study did not seem to have better understanding of technological systems than school pupils and teachers in previous studies, although this is in line with previous investigations on the general system thinking capabilities of children and adults. The most important implication of this study is that students need to be trained in systems thinking, particularly regarding how components work and connect to each other, flows (especially of information), system dependency, and the human role in technological systems.