Why do only a few food processing SMEs take advantage of lean manufacturing? Is there anything inherent to food processing SMEs with respect to plant, product, process and organizational behavior influencing the applicability and effectiveness of lean manufacturing? In other words: What are the determining factors that contribute to the variations in operational performance in food processing SMEs and most importantly, how? This doctoral research provides some interesting insights into this topic. Firstly, food processing SMEs are mainly focusing on quality assurance (food safety) and less on quality improvement. Secondly, lean manufacturing implementation improves the operational performance, especially in relation to productivity and quality. Thirdly, variations in the use of lean manufacturing practices are substantial and some practices are yet to be fully used in the food sector. Fourthly, the size of the company is positively correlated with the degree of use of lean practices. Fifthly, the commitment of the top management, training, change agent, product and process characteristics of the food sector are critical for the success of lean manufacturing implementation. Food processing SMEs that manage these determining factors effectively have a higher probability of implementation success. Finally, a framework - house of lean for food SMEs - that takes into consideration the needs and characteristics of food processing SMEs has been proposed in order to assist managers in lean practices implementation.