Placing reinforced concrete jackets or layers to strengthen or repair and strengthen concrete columns is a normal construction practice but there are many unresolved issues regarding the capacity of the strengthened elements. In the absence of any guidance, engineering judgement is often used. This paper sets out to assist the engineer when considering some of these unresolved issues. Revised values for factors of safety are proposed for design. A procedure to guarantee a sufficient connection between contact surfaces and to determine the performance of retrofitted columns is presented, considering the strengthened columns as “composite” elements. The parameters affecting the main mechanisms for the transfer of shear stress at the interface between new and old concrete are described and practical design considerations are given. An approximate procedure is presented, based on the design of monolithic elements, supplemented by the use of specific modification factors (monolithic factors), in order to evaluate the capacity of a strengthened element. Available experimental results are processed to derive appropriate values for monolithic behaviour factors and an extended analytical analysis is used to fill in gaps in the experimental work. Although this paper has particular relevance to seismic strengthening, its contents will have a wider application to strengthening in general. The object of this paper is to provide guidance so that the engineer is better equipped to deal with the practical design needs of today.