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The research work in this thesis is broadly divided into three parts: The first part deals with the pre-yield and post-yield behaviour of concrete columns. The object of this study is to determine the lateral force - displacement characteristics of reinforced concrete columns when subjected to constant axial load and to a monotonically increasing lateral deformation to failure. Analytical procedures are developed which predict the first quarter cycle of the lateral force-deflection envelope curve. The analytical results are then compared with tests results to confirm the validity of the method of analysis. In the second part of the investigation a method of analysis is developed which predicts the lateral force-displacement characteristics of concrete columns subjected to cyclic loading. This part is followed by a section dealing with the establishment of a criterion of collapse of concrete columns under repeated loading. In order to establish the validity and limitations of this method of analysis, and the criterion of collapse we compared the theoretical and experimental results of forty tests. Each column had encastre' ends and the axial load was held constant througholit the test. The main parameters which were varied throughout the test series were: the axial load, the longitudinal reinforcement, the dimensions of the columns, the pattern and rate of lateral cyclic loading. The third part consists of theoretical investigations and numerical evaluations of the response of reinforced concrete columns subjected to earthquake motions. The load restoring characteristic of concrete columns is represented by a non-linear model based on experimental evidence. The model has stiffness deteriorating property and includes the effect of gravity. Six components of the strongest ground motions yet recorded are used in a response analysis. When columns are subjected to real earthquake ground motions it is found that:- a) The damage potential or 'severity' of earthquake ground motions with regard to their effects on concrete columns in the period range of 0.4 to 2.0 seconds is directly related to maximum ground velocity, and the 'severity' of the earthquakes with regard to their effect on concrete columns in the short period range of 0.1 to 0.4 seconds is related to maximum ground acceleration. b) The duration of ground motion is responsible mostly for the failure of columns which cannot withstand repeated cyclic distortion. c) For preliminary design of concrete columns in the absence of any ground motion record a design coefficient is suggested in terms of the expected maximum ground velocity and acceleration as well as parameters which define the lateral force deflection relationship of the column. d) Study of the variation of 'time of collapse' with natural period of the column has revealed that: when the period of the columns are within the period range of 0.1 to 0.6 seconds there exists a marked dependency of time of collapse upon the natural period of the columns. In the period range of 0.6 to 2.0 seconds no marked dependency of the time of collapse on the natural period of columns is noted. Finally the relevant items required for design of columns are discussed: 1. The maximum value of relative displacement response. 2. The number of times that post-yield cyclic deformation is likely to occur. 3. The value of the permanent set. 4. The process of damage and loss of stiffness resulting from inelastic deformation during the earthquake.