Wearable activity monitors represent one of the most promising tools for the highly valid assessment of physical activity, sedentary behaviour, and sleep (i.e., 24-hour movement behaviours). Despite their considerable potential for research, population surveillance, and clinical practice, their wider accessibility is limited by the relatively high cost of established research-grade devices. The aim of this student PUŠ project was to develop and validate an affordable activity monitor for the assessment of 24-hour movement behaviours. Within the project, we developed the tri-axial accelerometer sloPAL, featuring a battery life exceeding seven days, wireless data transfer, and a thin design that enables comfortable wear on the anterior thigh. Technical verification demonstrated minimal noise and raw signal zero-offset error, as well as excellent device reliability under dynamic conditions. A laboratory validation study involving 22 healthy adults showed that, when analysed using the ActiPASS software, the sloPAL provided highly valid estimates of sedentary behaviour and physical activity, with results that were identical to those obtained from the reference device activPAL. A pilot free-living validation study involving six participants further confirmed excellent agreement between the two devices in the assessment of sleep, sedentary behaviour, and total physical activity. The slightly lower estimates of higher-intensity physical activity recorded by the sloPAL compared with the activPAL are most likely attributable to the proprietary raw signal preprocessing algorithm implemented in the reference device. In addition to the development and validation of the activity monitor, the project contributed to a systematic review of the methodology of wearable activity monitors validation studies, an analysis of the market for existing research-grade devices, software solutions for processing raw accelerometer signals, methods for attaching wearables to the body, and the challenges and opportunities associated with the use of accelerometers. The project outcomes provide a solid foundation for the further application, refinement, and commercialisation of the developed device and represent an important step towards the broader vision of improving access to high-quality tools for the assessment of 24-hour movement behaviours. Such tools have the potential to support research, population surveillance, and clinical practice, ultimately contributing to more effective health promotion and disease prevention.