This paper discusses unregulated market-based electricity transmission investment by third parties as opposed to regulated investment by designated transmission system operators. The analysis is set against a European and Australian institutional background and focuses on interconnection of different systems. The paper explores four areas: economies of scale, market power, detrimental investment and risks. The analysis argues for restricting market-based investment to controllable flow (DC or FACTS) only. This is in line with what seems to take place in practice in Europe and Australia, it strikes a balance between pros and cons of market-based investment and draws a sharp line between regulated and unregulated investments.