More than ever business applications feel the need of embracing responsiveness at its finest, be flexible, scalable, and guarantee resilience. As a result, there is an increasing tendency in adopting reactive approaches which even though seem complex in the beginning, strike as necessary for the development process of the application in the long run. What are the core concepts of the reactive paradigm? How do I decide if it's the right tool for the job? What are the challenges when applied in real-life projects? These will be some of the questions we will answer in order to provide a good understanding of what reactive programming stands for, what reactive systems bring to the table, how to apply design principles in various projects, and how to create fully reactive architectures.