In the rapidly evolving virtual reality (VR) field, assessing video quality on head-mounted displays (HMDs) for 360° videos presents unique opportunities and challenges. As immersive multimedia becomes increasingly widespread, understanding how users perceive and evaluate the quality of 360° videos is essential. This thesis investigates the subjective quality assessment tests of 360° videos, examining how participants' VR experiences, viewing conditions, and exploration behaviors shape perceptions of quality. This thesis aims to perform subjective quality assessment tests to study and understand how participants perceive the quality of 360° videos on an HMD. The thesis starts with an extended summary of the field of subjective quality assessment for 360° videos, followed by eight key publications, and unfolds into three main parts.

The first part of the thesis focuses on data collection to establish ground truth. It includes a comprehensive survey of annotated 360° images and videos datasets related to subjective quality assessment. It also presents a set of datasets collected specifically for subjective quality assessment tests for 360° videos with different test methods and viewing conditions conducted as part of the research. The second part of the thesis investigates how varying levels of VR experience affect participants' video quality assessments. It compares two test methods, the absolute category rating (ACR) and the modified ACR (MACR) method, to evaluate 360° video quality. Furthermore, this part evaluates simulator sickness in participants viewing 360° video on an HMD and explores how their prior VR experience levels correlate with the occurrence of these symptoms. The third and final part of the thesis focuses on assessing viewing conditions and rating consistency. It involves conducting subjective quality assessment tests for 360° videos under different viewing conditions, such as standing and seated viewing, and providing a statistical analysis of the psychophysical and psychophysiological measures. This part also investigates the consistency of 360° video quality assessments through repeated subjective quality assessment tests under opportunity-limited conditions. It examines how quality assessments vary between the standing and seated viewing conditions and explores whether participants' subjective evaluations of 360° videos change over time or remain stable across repeated exposures.