This lecture introduces Fractional Factorial Designs, which are experimental designs that use a subset of runs from a full factorial design to save resources. It covers the types of designs, examples, and the importance of balanced designs in statistical analysis. The instructor explains how to reduce the number of runs while maintaining statistical properties and discusses the advantages and disadvantages of fractional factorials. The lecture also explores the concept of exploiting redundancies in interactions and the philosophy behind the use of fractional factorial designs to exploit redundancy in full factorial designs.