This book contains two surveys on modern research into non-regular Riemannian geometry. Its authors, Reshetnyak, Berestovskij, and Nikolaev are well-known researchers in this area. They treat two-dimensional Riemannian manifolds of bounded curvature and metric spaces whose curvature lies between two given constants. Both these topics have not been treated in monograph form. Hence the book will be immensely useful to graduate students and researchers in geometry, in particular Riemannian geometry.