Multimorbidity, the coexistence of multiple chronic diseases or conditions, poses a major challenge for health-care systems worldwide. Traditional research has largely relied on cross-sectional studies, offering limited insight into multimorbidity evolution over time. This Personal View advocates for a paradigm shift towards longitudinal approaches that capture multimorbidity trajectories. Tracking the sequence, pace, and severity of disease accumulation can enhance our understanding of underlying mechanisms, inform early interventions, and improve patient care. Drawing on expert discussions from an international workshop held in Bielefeld, Germany, in May, 2024, we outline key themes and findings to guide future research on the dynamic processes underlying multimorbidity trajectories. Specifically, we summarise previous work, examine the challenges and opportunities of existing data resources, and highlight priority areas for further investigation. Advancing this field will require the standardisation of longitudinal multimorbidity phenotypes, integration of health and social care processes, and testing the usefulness of trajectories for patient-relevant outcomes and risk stratification. Progress will also depend on methodological innovation, patient and public involvement, harmonisation of diverse data sources, and close interdisciplinary collaboration. Ultimately, a trajectory-based framework for multimorbidity research can enable more personalised, efficient, and equitable health-care strategies, improving outcomes in ageing populations.