The interaction of relativistically intense laser radiation with plasma is rich in nonlinear processes like laser pulse self-modulation and self-focusing, wake-field excitation, wave-breaking and the creation of relativistic solitary structures. The creation and evolution of solitary structures during relativistic laser-plasma-interaction is discussed in this book. A relativistic Maxwell-fluid model is formulated to investigate these solitary structures. We discuss the creation of pre-solitons and their transition to post-solitons. The longitudinal and transversal stability of stationary soliton solutions to the Maxwell-fluid model are examined by numerical linear stability analysis. The transversal instability is identified to be the dominant process. Relativistic electrostatic wave-breaking often is part of the nonlinear evolution of unstable relativistic solitons. A process that will eventually lead to wave-breaking for every electrostatic wave in a cold relativistic plasma is presented. The formalism allows a generalization of known criteria to the relativistic regime and to estimate the breaking time.