Purchasing has become an increasingly important function in manufacturing companies, as it accounts for a large extent of total spending and influences a company's competitive position. It is relevant for Tetra Pak, a global manufacturer of packaging and processing solutions, active since the 1950s. Tetra Pak has over 100.000 active units at customers site over the world, to service them and offer spare parts is both a significant source of revenue for Tetra Pak and a service requested by the customers. The customers expect fast responses, as breakdowns in the food industry have severe consequences. It implicates that a wide array of parts are needed to be made available with short notice globally. To achieve it, Tetra Pak is working with an adapted consignment stock strategy, referred to as the logistics agreement. It means that some suppliers hold materials in finished stock. It enables the suppliers to deliver with short lead times, with the condition that if Tetra Pak does not purchase the material according to forecast, Tetra Pak financially compensates the suppliers for the remaining stock. The purpose of the thesis was to understand how the logistics agreement worked, but also to understand the implications of the solution and determine if the strategy was beneficial to Tetra Pak. Further, the aim was to compare Tetra Pak's processes with literature and comparative companies and give improvement suggestions. The research was initiated with a literature review to compile knowledge from research within purchasing, after-sales services, and spare parts management. Based on that, an analysis model was created. The model helped to analyze the empirical data by dividing the area into three different parts: the overall goals with the strategy, which spare parts should be included in the strategy, and which suppliers should be included in the cooperation. The empirical data was collected mainly through interviews and quantitative analysis based on historical informat, Implementing and applying buyer- supplier agreements to achieve increased performance in the after-sales service business of selling spare parts can be a valuable tool. Succeeding with it may be challenging. Due to the nature of spare parts, planning and overseeing the supply chain is quite tricky. To know when a machine will break down and need a spare part is almost impossible. The solution is to have the spare parts in stock to offer them to customers in need quickly. As some parts may take months to manufacture and each hour of downtime for the customer is costly, this is important. Keeping all the necessary stock in your warehouse is expensive, though. Parts Supply Chain at Tetra Pak uses a logistics agreement with their suppliers to tackle this, based on the consignment stock strategy. That means that they allow their suppliers to build up a level of pre-made stock for certain critical materials. The levels in Tetra Pak’s case are called Approved Stock Levels (ASL). Instead of having lead times of many months, they can decrease it to 14 days. Tetra Pak then takes on the financial risk for the ASL materials, agreeing to buy out pre-made stock that is not ordered during the set-out time period. The logistics agreement has been shown to improve the performance of the after-sales business at Tetra Pak. The company sees lower lead times, higher delivery performance and better availability of the ASL materials. The suppliers also showed during the study that they appreciate this way of working, as it gives them financial security, increased information sharing, and opportunities to optimize their production planning better. It also helps with improving the collaboration between supplier and buyer. The logistics agreement is not perfect, though, and there are areas to improve. Materials are each month selected to the list of ASL if their economic order quantity period is between 10 and 50 days. It has led to high volatility and turnover levels, making it hard for the