Introduction. Physical therapists (PTs) have the autonomy and expertise to assume a more significant role in the primary care of musculoskeletal conditions, validate diagnoses, and serve as a referral source to appropriate health care providers. Ultrasound diagnostic imaging has been identified as a high-priority area to advance science and innovation in physical therapy. Yet, few PTs are certified to incorporate musculoskeletal ultrasound (MSK-US) as a diagnostic tool into their personal scope of practice. Diagnostic MSK-US has unique benefits compared with other imaging modalities, and recent technological advances have reduced its cost and improved portability. However, no research exists describing the learning experiences and decision-making process of PTs who use MSK-US for diagnostic purposes. In addition, the educational process for learning MSK-US is not standardized. The study's purpose was to describe the learning and practice-based use of MSK-US by PTs registered in musculoskeletal sonography (RMSK).Methods.Using purposive sampling, we attempted to recruit all 21 RMSK-certified PTs currently using diagnostic ultrasound in clinical practice across the United States. Sixteen PTs participated in the study. We employed a qualitative, multiple case study, phenomenological approach. Data were collected using an online demographic survey and one-on-one, semi-structured interviews.Results.Sixteen interviews were conducted with RMSK-certified PTs. Data synthesis resulted in 5 elements: 1) self-directed learning; 2) educational process; 3) honing the skill and the role of mentorship; 4) diagnostic information; and 5) clinical application.Discussion and Conclusion.Participants identified elements critical for post-entry level, life-long, applied learning and integrating diagnostic MSK-US into clinical practice. The participant learning process was self-directed and incorporated various materials and methods to improve diagnostic skills. Participants honed their skills through repetition and one-on-one mentorship. Supportive employment settings were integral for creating environments conducive to learning and integrating MSK-US into clinical practice. Our participants perceived that ultrasound imaging provided them with the missing visual dimension used to improve diagnostic capabilities, supporting the benefits of direct access. © 2022 Journal of Physical Therapy Education. All right reserved.