This is the first exposition of the theory of quasi-symmetric designs, that is, combinatorial designs with at most two block intersection numbers. The authors aim to bring out the interaction among designs, finite geometries, and strongly regular graphs. The book starts with basic, classical material on designs and strongly regular graphs and continues with a discussion of some important results on quasi-symmetric designs. The later chapters include a combinatorial construction of the Witt designs from the projective plane of order four, recent results dealing with a structural study of designs resulting from Cameron's classification theory on extensions of symmetric designs, and results on the classification problem of quasi-symmetric designs. The final chapter presents connections to coding theory.