The heterogeneity of the population and the long, symptom-free intervals constrain the ability to generate data applicable across the population of ACHD or to adults with specific lesions or repairs. Despite the difficulty in studying ACHD populations, there is a growing body of high-quality data in these patients to guide the care of this relatively new population and, whenever feasible, these data were used to develop recommendations. Recommenda- tions are made based on the available data; however, when important clinical issues lacked data, first prin- ciples, extrapolation from data in other populations, and expert consensus are used to guide care. Patients with ACHD may have concomitant disease to which other existing guidelines apply, such as coronary ar- tery disease, HF, and arrhythmias. The data from ac- quired heart disease populations may apply to some patients with ACHD, and those circumstances are acknowledged in these recommendations and refer- enced accordingly. Patients with ACHD who are cared for in ACHD cen- ters have better outcomes than those cared for in cen- ters without ACHD expertise,S1.4-9 and this need for spe- cialized care is noted throughout the guideline. These recommendations are intended to provide guidance to a wide variety of providers caring for patients with ACHD, including general, pediatric, and ACHD cardiol- ogists, as well as surgeons, primary care providers, and other healthcare providers. In developing the 2018 ACHD guideline, the writing committee reviewed previously published guidelines and related scientific statements. Table 2 contains a list of publications and scientific statements deemed per- tinent to this writing effort; it is intended for use as a CLINICAL STATEMENTS AND GUIDELINES Table 2. 