The article describes the methods and rationale behind the development of the SPIRIT 2024 and CONSORT 2024 reporting guidelines. The authors aimed to harmonize the two guidelines, reflecting developments in evidence and best practices, and to simplify the user experience by incorporating content from key extensions. They conducted a scoping review to identify suggestions for changes to SPIRIT 2013 and CONSORT 2010, and compiled a list of potential modifications. These were presented to stakeholders for feedback in an international online Delphi survey. The results were discussed at an expert consensus meeting, where participants agreed on implementing 11/14 of the proposed changes that reached consensus in the Delphi survey and supported implementing 4/21 additional changes. The final SPIRIT 2024 and CONSORT 2024 checklists aim to provide updated, harmonized guidance for reporting randomized controlled trial protocols and results, respectively. The development process involved a literature review, Delphi consultation, expert consensus meeting, and writing meeting, ensuring broad stakeholder input and transparency.The article describes the methods and rationale behind the development of the SPIRIT 2024 and CONSORT 2024 reporting guidelines. The authors aimed to harmonize the two guidelines, reflecting developments in evidence and best practices, and to simplify the user experience by incorporating content from key extensions. They conducted a scoping review to identify suggestions for changes to SPIRIT 2013 and CONSORT 2010, and compiled a list of potential modifications. These were presented to stakeholders for feedback in an international online Delphi survey. The results were discussed at an expert consensus meeting, where participants agreed on implementing 11/14 of the proposed changes that reached consensus in the Delphi survey and supported implementing 4/21 additional changes. The final SPIRIT 2024 and CONSORT 2024 checklists aim to provide updated, harmonized guidance for reporting randomized controlled trial protocols and results, respectively. The development process involved a literature review, Delphi consultation, expert consensus meeting, and writing meeting, ensuring broad stakeholder input and transparency.