In a world where knowledge is a powerful force for positive change, Open Science is a catalyst for progress. It is a collective effort to ensure that knowledge is not confined to the few but shared with the many, transcending boundaries, and driving innovation for the betterment of society and our planet. Embracing Open Science is a step toward a more equitable, transparent, and prosperous future for all. Open Science holds particular importance for Ukraine, especially in the context of the ongoing war and post-war circumstances. Here are some specific reasons why Open Science can be crucial for Ukraine at this time: 1. International Collaboration and Support: Ukraine's scientific community can benefit from international collaboration facilitated by Open Science. By sharing research findings, data, and resources openly, Ukrainian researchers can access a global network of experts, funding opportunities, and collaborative projects that can help rebuild and strengthen their scientific infrastructure. 2. Access to Knowledge: In a post-war context, access to knowledge becomes critical for addressing a wide range of challenges, from healthcare to infrastructure rebuilding. Open Science ensures that essential information and research are freely available, making it easier for Ukrainian scientists and policymakers to access valuable insights and data. 3. Innovation and Recovery: Open Science can contribute to the country's recovery and development. It enables Ukrainian researchers to tap into global knowledge to develop innovative solutions for post-war reconstruction, economic growth, and sustainable development. 4. Human Capital Development: Encouraging Open Science can help retain and attract talent in Ukraine. Open access to educational resources and research opportunities can make it more appealing for young scientists and students to stay or return to Ukraine, contributing to the country's scientific and economic growth. 5. Data for Decision-Making: In a post-war scenario, data-driven decision-making is vital. Open Science provides access to essential data that can inform policies and strategies for rebuilding infrastructure, healthcare, and other critical sectors. 6. Transparency and Accountability: Open Science promotes transparency and accountability, which are essential for effective governance and combating corruption. It can help ensure that resources allocated for post-war recovery are used efficiently and for the benefit of the population. 7. Resilience and Preparedness: Open Science can help Ukraine better prepare for and respond to future crises, whether they be natural disasters or geopolitical challenges. By openly sharing knowledge and research, Ukraine can build resilience and adaptability into its scientific and educational systems. This introductory guide to Open Science is a joint effort of librarians from the Charles University in Prague (Czech Republic) and the University of Warsaw (Poland). It is our small intellectual contribution to, we hope, rebuilding Ukraine as soon as possible and strengthening its research potential. Authors of chapters: Valentýna Drtinová (Валентина Дртінова), Charles University - Chapters 2, 7, 8 Dagmar Hanzlíková (Даґмар Ганзлікова), Charles University - Chapter 4 Milan Janíček (Мілан Янічек), Charles University - Chapters 1, 3 Anna Książczak-Gronowska (Анна Ксьонжчак-Ґроновська), University of Warsaw - Chapter 12 Karolina Minch (Кароліна Мінх), University of Warsaw - Chapters 9, 10 Lilianna Nalewajska (Ліліанна Налевайська), University of Warsaw - Chapters 5, 11 Zuza Wiorogórska (Зуза Вьороґурська), University of Warsaw - Chapter 6 Anna Wołodko (Анна Володко), University of Warsaw - Chapter 13Translation from English: Kateryna Tkachuk Typesetting and layout: Justyna Grzymala-Luszcz This work was financed by Polish National Agency for Academic Exchange (NAWA). NAWA 'Solidarity with Ukraine' programme:https://nawa.gov.pl/instytucje/uniwersytety-europejskie „Solidarity with Ukraine – 4EU+ for Ukraine” is a programme of the Polish National Agency for Academic Exchange (NAWA) with a budget of approx. 765 200,00 PLN (approx. 162 000 euros). University of Warsaw is its coordinating institution. Project lifecycle: 9 January 2023 – 31 December 2023 http://en.bwz.uw.edu.pl/solidarity-with-ukraine-4eu-for-ukraine-about-the-project/