Host cell protein (HCP) is a critical quality attribute of biologics. HCP presence in biologic drugs is a patient safety risk, potentially resulting in reduced drug efficacy or immunogenic response. It is essential to challenge HCP assays from the beginning of development for each new molecule. Here, we highlight the current challenges our customers are facing regarding their assays on both project and supplier levels. What kind of assays are available today and what are the strengths and weaknesses? How can Cytiva support customers and resolve their challenges? We will discuss strategies you can adopt to minimize the risk of unexpected HCP levels later in development.