The National Research Council (NRC) has been conducting decadal surveys in the Earth and space sciences since 1964, and released the latest five surveys in the past 5 years, four of which were only completed in the past 3 years. Lessons Learned in Decadal Planning in Space Science is the summary of a workshop held in response to unforseen challenges that arose in the implementation of the recommendations of the decadal surveys. This report takes a closer look at the decadal survey process and how to improve this essential tool for strategic planning in the Earth and space sciences. Workshop moderators, panelists, and participants lifted up the hood on the decadal survey process and scrutinized every element of the decadal surveys to determine what lessons can be gleaned from recent experiences and applied to the design and execution of future decadal surveys. Table of Contents Front Matter 1 Summary of Keynote Speakers Remarks 2 Overview of the Decadal Survey Process 3 Decadal Survey Chairs' Perspective 4 Sponsor Agency Perspectives 5 Decadal Survey Program Formulation and Opportunities for Improvement 6 The Role of Cost Estimates, Technical Evaluations, and Budget Projections in Prioritizing Missions 7 How to Plan for High-Profile Missions 8 Incorporating International Perspectives in Future Decadal Planning 9 Decadal Survey Stewardship: The Role of the Mid-Decade Reviews and Standing Committees 10 Concluding Remarks Appendixes Appendix A: Workshop Agenda Appendix B: Biographies of Planning Committee Members, Moderators, Panelists, and Staff