Context: The Veterans Health Administration (VA) National Center for Ethics in Healthcare implemented the Life-Sustaining Treatment Decisions Initiative, including policy and practice standards, clinician communication training, a documentation template, and central implementation support to foster advance care planning via goals-of-care conversations for seriously ill veterans in 2014, spreading nationally to other Veterans Health Affairs (VA) sites in 2017. Objectives: Our goal was to describe the range of early implementation experiences among the pilot sites, and compare them with spread sites that implemented LSTDI about two years later, identifying cross-site best practices and pitfalls. Methods: We conducted semistructured interviews with 32 key stakeholders from 12 sites to identify cross-site best practices and pitfalls related to implementation. Results: Three primary implementation themes emerged: organizational readiness for transformation, importance of champions, and time and resources needed to achieve implementation. Each theme's barriers and facilitators highlighted variability in success based on complexity in terms of vertical hierarchy and horizontal cross-role/cross-clinic relationships. Conclusion: Learning health care systems need multilevel interdisciplinary implementation approaches to support communication about serious illness, from broad-based system-level training and education to build communication skills, to focusing on characteristics of successful individual champions who listen to critics and are tenacious in addressing concerns.