Affiliate marketing programs have emerged as one of the fastest-growing methods for online retailers to acquire customers and increase sales. Affiliate marketing offers a number of advantages, including a relatively low cost and the ability to accurately track the actions of website visitors and their responses to targeted promotional activities. However, while these programs have proven effective in increasing website traffic and sales, illegal or inappropriate activities on the part of affiliates could negatively impact a retailer’s brand in the eyes of customers. This study reviews the stated guidelines in one-to-many affiliate programs in the three major affiliate networks in Spain as a first step in understanding how online retailers control the business models and promotional tools used by their affiliates. The conclusion is that there is a significant lack of transparency in the guidance and restrictions communicated to affiliates, which increases the risk of inappropriate behavior or misconduct. Consequently, affiliate monitoring by online retailers becomes increasingly important. General recommendations to improve monitoring are considered.