This paper provides a technical overview of the on-line lightning monitoring system (OLMS), a sophisticated second-generation system designed to monitor the effects of lightning-induced electromagnetic (EM) transients on ground-based space systems. The potential damage of sensitive electronic systems by lightning-induced EM transients is of concern to system designers, operators, and maintainers. The OLMS uses broadband sensors to measure free-field radiation (electric and magnetic fields), as well as energy coupled to conductors (current and voltage). Characterization of the measured signals is performed by two subsystems in parallel: an analog transient processor (TPM) and conventional state-of-the-art digitizers. While the digitizers are able to provide the actual high-frequency waveforms desired by many users, they have inherent limitations in duty cycle and large data storage requirements. The companion TPM electronics processes the data in real time to extract parameters of interest, such as peak amplitudes and energy, all in the analog domain and with full signal bandwidth. Unlike digitizer-based systems, the TPM is guaranteed never to miss a transient event and produces a very compact result data stream. The data from the OLMS are available in real time and are accessed by users via the Internet, using a standard web browser interface. The OLMS system is being developed to support spacecraft launches at a number of launch sites in the United States. The OLMS will allow users to monitor accurately the EM threat to their sensitive electronic systems and retest systems only if necessary. This is of value since overtesting not only leads to very expensive delays but also affects schedules, while undertesting may result in the deployment of a damaged system where repair may not be possible.