In this project, the buckling of imperfect circular cylindrical shells under uniform axial compression has been investigated. The imperfection considered is prismatic and is in the form of flat spots along the complete length of the shell. The problem is solved by considering it as an interaction problem between curved and flat panels. Shell equations are satisfied in the curved portions of the shell while flat plate equations are used in the flat spot regions. At the common edge between two adjacent panels, forces and displacements are matched to arrive at the eigenvalue problem for the critical load of the shell. Two flat spot configurations have been studied. In the first case, a single flat spot along the complete length of the shell is considered. Curved and flat panels join to form sharp corners at their common edges. Numerical results are presented to show the effect of the width of the flat spot, thickness ratio and length to radius ratio of the shell, on the buckling load of the shell. In the second case, the imperfection is in the form of two or more identical flat spots distributed uniformly along the circumference of the shell. Between two consecutive flat spots is a uniform radius cylindrical panel joining smoothly to the flat panels. This analysis is valid for any integral number of identical flat spots. Numerical results are presented for 2, 3, 4, 8-flat spots. The effect of width of flat spots, radius of curved panels, thickness ratio of the shell has been investigated. The results, presented in this paper, clearly demonstrate a very significant reduction in the buckling load of the shell below its classical value. This is true even in the presence of relatively small flat spots. This reduction becomes more severe with a decrease in the thickness of the shell or an increase in the width of the flat spot.