In this chapter, sample collection and necropsy procedures for fish are dealt with in detail. Ideally, these will prove useful guide documents for aquaculture practitioners and researchers alike. The introduction discusses the importance of aquatic animal health as it relates to the economic and ecological effects of diseases in aquaculture systems. Principles of disease diagnosis are outlined in the chapter, with a focus on fish bacteriology, virology, parasitology, and general necropsy. Systematic approaches for the identification and management of diseases are thus prominently featured in the module. Standard procedures for necropsy of finfish are detailed and involve step-by-step instructions for external and internal examinations, taking tissue samples, and preserving them. Techniques in histological analysis, including fixation, sectioning, and staining, are covered extensively to ensure excellent pathological assessments. The chapter also gives a set of practical directions on how to send samples and suggests that to prevent dislodging specimen integrity, the packaging and labeling must be proper. A template for fish pathology laboratory submission forms is provided for swift communication and reporting. This chapter, with respect to standardized protocols, ensures that diagnostic outcomes are reliable and reproducible, thereby affording support to the causes of disease management in aquaculture. The final section emphasizes the importance of proper necropsy and diagnostic practices in pushing forward fish health research and ensuring sustainable aquaculture. Such a resource indeed remains the well-rounded guide that professionals should use to enhance disease surveillance and improve the overall health and productivity within aquaculture systems.