Approximately three-quarters of regularly menstruating women experience premenstrual symptoms (Hobel et al., 2015; Safarzadeh et al., 2016). Symptoms vary in severity from mildly disturbing to severely disabling for some women and can be so severe that they interfere with daily functioning and personal relationships (Safarzadeh et al., 2016). A range of physical, psychological, and behavioural symptoms can occur in the late luteal phase of the menstrual cycle, with symptoms abating shortly after the onset of menses (Lee et al., 2022). Various pharmacological and non-pharmacological interventions have shown promising results in reducing premenstrual symptom burden (Itriyeva, 2022; Saglam & Orsal, 2020; Ussher & Perz, 2017). A growing body of evidence suggests lifestyle interventions targeting physical activity, healthy diet, and weight management can reduce the intensity of premenstrual symptoms (Esmaeilpour et al., 2019; Saglam & Orsal, 2020), but the effectiveness of multiple health behaviour change interventions (MHBC) in this population remains largely untested. Previous research in midlife women provides evidence that MHBC interventions are effective in reducing hormone-related symptoms of menopause (Anderson et al., 2015; McGuire et al., 2019). The prototype Younger Women's Wellness Program (YWWP), a 12-week structured MHBC program has been designed and developed for women aged 18-35 years (Anderson et al., 2018), but its effectiveness has not been established. The YWWP aims to change health behaviour and improve health literacy about a range of health issues pertinent to younger women, such as physical activity, weight management, healthy eating, smoking, alcohol consumption, stress management, sleep, and health screening behaviours. The YWWP comprises a self-directed PDF with a step-by-step weekly program, which includes detailed health information, activities and goal setting targeting these behaviours, and is underpinned by social cognitive theory (SCT). Given the likely benefits of health behaviour change on premenstrual syndrome (PMS), it was hypothesised that this intervention would reduce symptoms and enhance health and well-being. To augment this intervention, an mHealth period and premenstrual symptom tracker application was added to increase symptom awareness and improve menstrual health literacy. The Clue period tracker application (CPTA) is one such mHealth application that has shown positive results in increasing participants' awareness and understanding of their menstrual cycle and the timing of their premenstrual symptoms (Krauskopf, 2019). The CPTA also allows users to choose which indicators and symptoms they wish to track or ignore. These include menstrual bleeding, pain, cravings, emotions, energy levels, weight, sleep duration, and changes in digestion, hair, skin, bowel movements, cervical fluid, and body temperature. Self-efficacy is complex and multi-faceted (Museus et al., 2022), and YWWP and CPTA consider different aspects of health behaviour change and self-efficacy that are likely to enhance each other. Specifically, CPTA relates to menstrual health literacy, while YWWP is an MHBC program for improving health and well-being. Although these interventions in isolation might reduce premenstrual symptoms, their combined impact is largely unknown and requires further testing. Given the high impact of premenstrual symptoms, there is a need for rigorously designed non-pharmacological interventions to improve health and reduce the intensity of premenstrual symptoms in young women. Aims: The overarching aim of this two-phased study is to explore the feasibility and acceptability of a multiple health behaviour change intervention for managing premenstrual symptoms in university students aged 18-35 years. The specific aims for each study phase are as follows: Phase 1: - To establish feasibility through composite analysis of eligibility, recruitment, retention, protocol adherence, and missing data of a parallel three-group randomised pilot trial; - To explore the common premenstrual symptoms experienced by university students and explore the preliminary effectiveness of a 12-week intervention on the severity of premenstrual symptoms, physical activity, functional health, and well-being, sleep quality, and self-efficacy; - To establish sample size estimates for a fully powered randomised controlled trial (RCT). Phase 2: - To gain an in-depth understanding of the interventional components that were helpful and acceptable for women and those that were not; - To explore the content and delivery methods likely to yield the maximum likelihood of participant engagement and retention and support self-management of premenstrual symptoms; - To understand the impact of premenstrual symptoms on health-related quality of life and self-efficacy in women. [...]