Meinhard Kieser conveys the fundamental principles of sample size calculation using real examples and demonstrates their application. For the most common application scenarios, the corresponding sample size calculation formulas are derived. Beginners thus have the opportunity to learn and practice the basics of sample size planning. The statistical backgrounds of the formulas and more general relationships are also explained, along with guidance on what should be considered in every sample size calculation. This makes the Essential go far beyond a mere collection of formulas and serves as a valuable addition for individuals who are already engaged in medical research and have experience in sample size calculation.