Canine lymphoma is one of the most common cancers in dogs, and it originates from the uncontrolled proliferation of lymphocytes. Due to lymphoma having different subtypes, accurate diagnosis requires microscopic evaluation rather than reliance on clinical presentation alone. My capstone examines the histologic and psychology features used to distinguish reactive lymphoid hyperplasia from malignant lymphoma, and the role of immuno phenotyping in determining the origin of the tumor. Key diagnostic indicators include a monophonic population of lymphocytes, enlarged nuclei, prominent nucleoli, and increased methodic activity. Immunohistochemical markers such as CD3 and CD20 enable differentiation between T cell and B cell informer, which carry distinct prognosis implications. By explaining the current veterinary diagnostic approach, my capstone emphasizes the importance of microscopic classification in guiding prognosis, ultimately improving clinical outcomes in dogs.