Background: Legally recognized advance directives (ADs) have to be signed by the person to whom the decisions apply. In practice, however, there are also ADs written and signed by legal proxies (surrogates) on behalf of patients who lack decision-making capacity. Given their practical relevance and substantial ethical and legal implications, ADs by proxy (AD-Ps) have received surprisingly little scientific attention so far.Objectives: To study the form, content, validity, and applicability of AD-Ps among German nursing home residents and develop policy implications. Methods: Secondary analysis of two independent cross-sectional studies in three German cities, comprising 21 nursing homes and 1528 residents. The identified AD-Ps were analyzed in parallel by three independent raters. Interrater agreement was measured using free-marginal multi-rater kappa statistics.Results: Altogether, 46 AD-Ps were identified and pooled for analysis. On average (range), AD-Ps were 1 (1-7) year(s) old, 0.5 (0.25-4) pages long, signed by 1 (0-5) person, with evidence of legal proxy involvement in 35%, and signed by a physician in 20% of cases. Almost all the AD-Ps reviewed aimed to limit life-sustaining treatment (LST), but had widely varying content and ethical justifications, including references to earlier statements (30%) or actual behavior (11%). The most frequent explicit directives were: do-not-hospitalize (67%), do-not-tube-feed (37%), do-notattempt-resuscitation (20%), and the general exclusion of any LST (28%). Inter-rater agreement was mostly moderate (kappa ≥0.6) or strong (kappa ≥0.8).Conclusions: Although AD-Ps are an empirical reality in German nursing homes, formal standards for such directives are lacking and their ethical justification based on substituted judgment or best interest standard often remains unclear. A qualified advance care planning process and corresponding documentation are required in order to safeguard the appropriate use of this important instrument and ensure adherence to ethico-legal standards.See related editorial by Cohen et al.