Background: Breast cancer survivors undergoing adjuvant endocrine therapy frequently experience loss of lean mass, declines in physical function, and cancer-related fatigue. Supervised exercise training can improve body composition, muscle strength, and quality of life, but adherence to in-person programs is often limited by logistical barriers. Remotely supervised, real-time online exercise programs have been proposed as a more accessible alternative; however, their effectiveness and acceptability in this specific treatment phase remain insufficiently established. Objectives: To compare body composition, muscle strength, cardiorespiratory capacity, fatigue and quality of life in breast cancer survivors receiving adjuvant endocrine therapy who completed a 12-week home-based physical training program delivered in two formats — remotely supervised (live online) or unsupervised (exercise instruction only) — and to assess reasons for declining the training program. Methods: This thesis comprises two components of a randomized clinical study. In the first, we investigated the effects of a 12-week home-based physical training protocol delivered in two formats: (i) remotely supervised in real time and (ii) unsupervised. In the second component, we evaluated the characteristics of, and reasons reported by, women who declined to participate in the intervention. Eligible participants were women aged ≤65 years with luminal breast cancer, currently receiving adjuvant endocrine therapy with tamoxifen or aromatase inhibitors initiated 6 to 36 months prior to screening, and previously treated with neoadjuvant or adjuvant chemotherapy and surgery, with or without radiotherapy. Ninety-eight women were identified through electronic medical records between May 2023 and February 2025. Of the 73 women who responded to telephone or WhatsApp contact, 43 agreed to enroll in the randomized clinical study; 4 were subsequently excluded. Of the 30 women who declined to participate in the training program, 21 agreed to complete questionnaires. In the randomized study, 39 women on adjuvant endocrine therapy were allocated to either the supervised online group (SOG; n=20) or the non-supervised group (NSG; n=19) for 12 weeks. Lean mass (primary outcome), body composition, muscle strength, peak oxygen consumption (VO₂peak), and quality of life were assessed before and after the intervention. In the second study, clinical data, physical activity level (IPAQ), fatigue (FACT-F), and quality of life (FACT-B) from the 21 women who completed questionnaires were compared with data from the 39 women included in the randomized study. Reasons for declining were also analyzed. Results: In the randomized study, significant group × time interactions were observed for lean mass, fat mass, and lower-limb strength. Only the supervised online group showed a significant increase in lean mass (+1.05 kg), a reduction in body fat percentage (−1.70%), and a greater gain in lower-limb strength. No significant increase in VO₂peak was observed in either group. Quality of life improved in both groups. In the second study, women who declined participation in the training program reported higher physical activity levels and lower fatigue compared with those who participated. The most frequently cited reasons for declining were already being physically active (52%) and lack of time to attend in-person assessments (33%). Conclusion: Remotely supervised online exercise training led to significant improvements in body composition (increased lean mass and reduced fat), lower-limb strength, quality of life, and fatigue. In the unsupervised group, no significant improvements in body composition were observed, although significant improvements in lower-body strength, quality of life, and fatigue also occurred. Although between-group differences were not significant, participants in the supervised group were more likely to achieve clinically meaningful improvements. These findings indicate that online supervised exercise is a feasible and effective approach to mitigate treatment-related declines and to integrate structured exercise into survivorship care. Declining participation was primarily associated with the perception of already being sufficiently active and with time constraints related to in-person baseline and follow-up assessments. Flexible assessment logistics and personalized recruitment strategies may increase engagement in exercise programs among breast cancer survivors. For women who already engage in regular physical activity, acknowledging their ongoing routines and offering structured monitoring rather than full program enrollment may help sustain benefits over time.