Semi-rigid connections are now-a-days widely employed by steel structures. Such connections take better advantage of the overall structure capabilities. In this work the properties and classification of semi-rigid connections is discussed. Additionally numerical and analytical models for prediction of connection behavior are presented. A finite element based methodology for the numerical evaluation of moment-rotation relations and load carrying capacity of semi-rigid connections is proposed. An important feature of this model is a complete a three dimensional geometrical description of the many components of the connection, such as plates, column, beam, bolts and nuts and the interaction between them. The extension of beam and column to be modeled is determined by numerical calibration. Frictionless contact between the connection components is considered by a specific contact algorithm which ensures non- penetration and permits separation of the individual parts. In this work a solution strategy for the contact problem based on the establishment of a linear complementary problem is adopted. The three dimensional modeling is carried out by a mesh of hybrid hexahedral eight node elements, which allow the use of relatively coarse meshes. Effects such as the presence of large deformations, yielding of components and pre- stressing of bolts are present in the model. The proposed model is applied to the analysis of the behavior of extended end-plate connections and web cleat connections. The numerical results are compared to experimental data for model validation. One of the objectives of this analysis is the evaluation of the participation of each component on the overall flexibility of the connection.