This paper discusses a number of aspects of operational-risk assessment of transmission systems. perational-risk assessment uses techniques developed from reliability analysis to predict the performance of the transmission system during a short period of time (several hours) starting from a well-defined operational state. The influence of weather parameters on the momentary failure rate and the operational risk is discussed in this paper as well. The operational-risk method is illustrated by means of a simple but realistic transmission-system model. The paper also discusses some of the applications for such methods in operation and planning of transmission systems.