Let's take a journey through the evolution of production systems, starting with the early 1900s. Henry Ford was a pioneer of mass production, using conveyor belts and division of labor to churn out large quantities of standardized products. This method, however, was a push-based approach and had its limitations, particularly when it came to variety and customization.

Fast forward to post-WWII Japan, where the Toyota Production System was born. This system was a game-changer, combining principles like just-in-time, jidoka, a form of automation, and continuous improvement. These principles allowed for low inventories, even with a high variety of products. A 1990 MIT study coined the term "lean" to describe the Toyota Production System, highlighting Toyota's superior productivity and quality. This lean approach soon spread worldwide, with other automakers creating their own production systems inspired by TPS.

It's important to understand that lean is more than just a set of tools—it's an integrated system. This means it requires a harmonious alignment of strategy, principles, culture, and methods. Two key pillars across all lean systems are quality and just-in-time production. Implementing lean is a step-by-step process that focuses on stability, flow, takt (the rate at which a product needs to be produced to meet customer demand), pull (where production is based on actual demand), and perfection. The ultimate goal of lean is to have zero defects, 100% value-add, one-piece flow (where products are produced and moved one unit at a time), and to prioritize employee welfare.

Lean is a departure from traditional push systems that rely heavily on forecasts. It allows for customization and variety with minimal inventory by using techniques like single-minute exchange of die (a method for reducing setup times), kanban (a scheduling system), and heijunka (production leveling).