This book provides an introduction to classical integral geometry and its practical applications in stereology and image reconstruction. It primarily presents the elementary two-dimensional integral geometry and its practically significant generalization to the three-dimensional case. Key topics include the fundamentals of convex geometry, integral geometry, and the Radon transform. Numerous illustrative examples and applications demonstrate the fundamental importance of the field for image analysis in biology, medicine, petrography, and metallurgy. Each chapter includes exercises (with solutions at the end of the book) to help reinforce the material learned. Extensive biographical notes on integral geometers and stereologists complete the book.