What is just-in-time (JIT) inventory?

JIT inverts the default instinct to stockpile. Instead of buffering against uncertainty with inventory, it shrinks stock toward what immediate demand requires and relies on frequent, reliable replenishment to keep the line fed. Toyota built the model: parts arrive at the work cell hours before use, not weeks. The prize is everything carrying cost was eating: freed cash, freed space, less obsolescence. There's also a side effect lean practitioners value most. With no buffer to hide behind, process problems surface immediately. The dependency is the strategy's whole risk profile. JIT assumes short, stable lead times, suppliers who deliver on schedule, and demand that's forecastable at the daily level. When any of those breaks, there's no cushion, which is why supply shocks turn JIT operations into case studies. The post-2020 consensus isn't that JIT failed but that it needs sorting: run lean on items with reliable local supply and steady demand, and hold deliberate safety stock on long-lead, single-source, or critical items. Pure JIT across an entire catalog is a bet that nothing will ever go wrong upstream. For a small operation, JIT isn't an all-or-nothing doctrine. It's a per-SKU question: for this item, with this supplier's actual lead-time record, how little can we hold without gambling a job on a delivery truck?