Policy interest in international surveys on Adult Learning (AL) has increased strongly. AL survey data are used as benchmarks for a country's educational system. However, results of key indicators like participation in learning activities often vary remarkably between different data sources. Stating that these differences are due to varying concepts and methods is not enough. The key question is: Which figures represent reality more appropriately? Therefore, evaluation of survey concepts and methods is crucial for international comparison of Adult Learning. This report provides guidelines on methodological and conceptual issues. Part one covers methodological aspects while part two deals with concepts, definitions and example questions. Recommendations are based on input from 14 countries. The methodological section covers data collection (telephone vs. f2f-interviews and online surveys); reference period (calendar year; recall problems); target population (age limit; non-national residents); sample design (proxy interviews, random selection of learning activities). The second part first discusses the basic concepts of Adult learning used in the European Adult Education Survey and in non-European countries (e.g. Canada, USA), including informal learning. The report then goes on to discuss empirical concepts and questions on AL participation and shows how different concepts affect empirical results and recommends example questions for formal, non-formal and informal learning. Other chapters refer to volume of AL; costs (what can be answered by non-experts?); providers of AL (define by teacher or learning location?); fields (ISCED, etc.); obstacles and benefits (which items are more valid?) and imputation. This report will act as a useful resource tool for researchers and policy makers when designing new national AL surveys or when optimising existing surveys. Results are focussed by a summary of conclusions, recommendations and example questions at the end of each section. Annexes include: (1) Acronyms used in this report; (2) References; (3) Existing OECD Education Working Papers. (Contains 40 footnotes and 18 figures.)