The proposed revisions of AC 20-136 and AC 20-53A by the SAE and EUROCAE lightning committees do not include the lightning environment parameters as has been the case in the previous version of these AC's. The committees have recently drafted a new “Aircraft Lightning Environment and Related Test Waveforms Standard” known as SAE-AE4L-97-4, the “Purple Book” in the U.S, and EUROCAE ED84 in Europe. This document contains a description of the phenomenology related to aircraft lightning interaction, and all the external parameters of the environment (i.e. current, statistics, repetition rate, etc.) and the associated representative waveforms. Moreover, the definitions of multiple burst and multiple stroke environment have been revisited based on a better understanding of the statistics of lightning. The document also provides definition and guidance in the choice of the induced transient related to the external environment and these are tabulated as a function of the system installation characteristics. This paper explains the main content of the document and relates some of the problems that have been encountered and solved during the drafting process.