Aerated stirred tank reactors are widely used in the biopharmaceutical industry for cell culture applications due to their flexibility and broad range of uses. This book chapter provides an overview of the main characterization methods for aerated stirred tank reactors in the context of cell culture processes. The methods described include mechanical power input, volumetric mass transfer, flow field, and mixing characterization. Examples are given using stirred tank reactors of different scales, ranging from a 3 L laboratory scale to a 15,000 L industrial scale. The chapter also discusses recent developments in the use of mobile, encapsulated sensors known as Lagrangian Sensor Particles (LSP) for continuous data collection and transmission throughout the entire reactor volume, providing insights from the perspective of cells during a cell culture process. Additionally, the state-of-the-art in numerical flow simulation is discussed, using aerated stirred tank reactors as an example to illustrate the challenges and opportunities offered by numerical flow simulation. It is highlighted that the methods described are specifically focused on cell culture applications, which have lower specific power inputs and aeration rates compared to other types of applications such as microbial fermentations or chemical reactions. The chapter also provides a brief overview of the fundamentals of aerated stirred tank reactors, including their design and components, and emphasizes the importance of equipment characterization for successful process transfer and scale-up in cell culture processes.