The objective of this study is to develop and test a novel mobile health (mHealth) behavioral intervention to enable Hematopoietic stem cell transplant (HCT) patients to cope with their symptoms to improve their ability to engage in physical activity that can improve physical disability. We developed a mHealth pain coping skills training program for HCT patients and found in a small trial (NCI R21) that better pain management provided some advantages for physical disability and activity. The next step in this research is to enhance our current coping skills program to address pain, fatigue, and psychological distress, and their interference with daily activity. Using the NIH Treatment Development Stage Model as a guide (Stage I), our interdisciplinary team will develop a hybrid in-person and mHealth HCT Coping Skills Training for Symptom Management and Daily Steps (CST Step-Up) intervention protocol. CST Step-Up will provide patients with cognitive behavioral coping skills training and activity coaching sessions to enhance their ability to cope with symptoms (pain, fatigue, distress) that interfere with physical activity. CST Step-Up will be developed by experts in symptom management, members of the HCT medical team, and with extensive input from HCT patients. Innovative features to be considered in CST Step-Up development include: in-person sessions prior to discharge with subsequent sessions occurring at home via video-conferencing; a study mobile application and wireless activity trackers to capture and transmit symptom and activity data to the study team in real-time with subsequent personalized feedback via push notification; cognitive behavioral strategies to decrease the impact of symptoms – pain, fatigue, distress – that most interfere with physical activity. Our central hypothesis is that this process will develop a CST Step-Up protocol that is feasible and acceptable and leads to reduced physical disability, as well as decreased symptom burden and increased physical activity. We will pursue three specific aims: 1) Fully develop the CST Step-Up protocol for HCT patients. Development will consist of 5 focus groups with HCT patients (n=6/group), a focus group with HCT providers (N=6), and user testing with 10 HCT patients. 2) Use a small randomized controlled trial (N=40) to examine feasibility and acceptability of the developed CST Step-Up protocol. Feasibility will be assessed via (a) study accrual, (b) protocol adherence, and (c) retention. 3) Examine outcome patterns suggesting the efficacy of the CST Step-Up protocol for improving physical disability and other outcomes compared to an HCT Education group. Confirmed hypotheses would provide the first demonstration of the feasibility, acceptability, and positive impact of a hybrid in-person and mHealth coping skills training and activity coaching intervention that reduces physical disability by concurrently and synergistically decreasing symptom burden and increasing physical activity. This project has the potential to lead to future research (R01) that can redesign existing modes of behavioral intervention delivery, improve continuity and coordination of care, and ultimately enhance patient outcomes.