Due to rising demands on productivity and flexibility, assembly processes are currently experiencing a substantial transformation. Workstations where humans and robots work closely together are becoming increasingly popular, as they provide major advantages compared to manual assembly and full automation. Yet, human-robot collaboration (HRC) can only be successful if the workforce is willing to accept it. How assembly workers perceive HRC still has to be properly investigated. An exploratory investigation using a Grounded Theory approach was conducted to identify factors that are likely to influence workers' acceptance of introducing HRC at work. Seventeen workers with various levels of HRC experience from four different manufacturing companies were interviewed. Findings reveal that some workers perceive HRC as a threat, while others regard it as an opportunity. This perception seems to depend both on their thoughts and feelings about the technology, i.e., collaborative robots, and the organizational change associated with the introduction of this technology. Several factors related to the robot (object-related factors), the individual background of the workers (subject-related factors), and the organizational environment (context-related factors) are found to influence workers' thoughts and feelings. Implications for researchers and manufacturing companies are outlined. CCS Concepts: • Human-centered computing → HCI theory, concepts and models ; Empirical studies in HCI ; User models; User studies; Field studies; Collaborative interaction; • Social and professional topics → Socio-technical systems; • Computer systems organization → Robotics ; • Social and professional topics → Computer supported cooperative work; Employment issues; • Applied computing → Psychology;