In this two-part paper, Michell's century-old stress-based single-load truss topology optimization paper is extended to multiple load conditions. In Part I, so-called 'optimal plastic design' of multi-load trusses is reviewed, which is based on ultimate (limit or collapse) load principles and requires only statical admissibility of the solution. However, its connections to 'optimal elastic design' will be explained, and these will be used in Part II for the full extension of Michell's theory to elastic multi-load problems, which will fill a significant gap in our knowledge.