Canadian nickel-copper ore deposits have been a major source of nickel, copper, cobalt and precious metals for more than 120 years. The two main minerals of interest, pentlandite (Pn, (Ni,Fe)9S8) and chalcopyrite (Cp, CuFeS2), are usually accompanied by large quantities of pyrrhotite (Po, Fe1-xS). Until the 1950s, Po containing small amounts of nickel was routinely smelted as part of the valuable Ni concentrate. The rapid growth in Ni demand following World War II created impetus to treat the Po separately. This action would liberate a valuable smelting capacity for higher value Pn and recover Ni, Fe, S and energy. The Ni industry would then have a more sustainable process. Both Inco (now Vale) and Falconbridge (now Glencore) developed processes and built large industrial plants for this purpose that operated with some success. But technical issues and tenuous economics were continual challenges. By the early 1980s, these industrial operations were closed and Po containing up to 1%Ni became a waste material reporting to the tailings stream. As part of a major study on possible processing methods of Po for both value recovery and waste remediation, this paper presents a historical perspective on Canadian Po tailings with regard to their inventory.