This book discusses some of the open questions addressed by researchers in general relativity. Photons and particles play important roles in the theoretical framework, since they are involved in analyzing and measuring gravitational fields and in constructing mathematical models of gravitational fields of various types. The authors highlight this aspect covering topics such as the construction of models of Bateman electromagnetic waves and analogous gravitational waves, the studies of gravitational radiation in presence of a cosmological constant and the gravitational compass or clock compass for providing an operational way of measuring a gravitational field. The book is meant for advanced students and young researchers in general relativity, who look for an updated text which covers in depth the calculations and, equally, takes on new challenges. The reader, along the learning path, is stimulated by provocative examples interspersed in the text that help to find novel representations of the uses of particles and photons.