Digital inline holography (DIH) is an imaging-based sensing approach that captures the interference patterns produced when light interacts with particles suspended in air or liquid within a three-dimensional sample volume. These holograms can be computationally reconstructed to recover particle size, morphology, three-dimensional position, optical properties, and dynamic behavior in situ. Compared with conventional microscopy, DIH does not require precise mechanical focusing and provides a much larger depth of field, enabling high-throughput measurements across extended sample volumes. The technique is label-free, involves simple sample preparation, and can be implemented in a compact optical system with stable calibration over long operating periods. When integrated with AI-aided preprocessing, detection, and particle characterization, DIH supports faster analysis and automated workflows, making it a powerful platform for in situ particle analysis in both industrial and academic applications.