Allergic contact dermatitis (ACD) is a type IV delayed hypersensitivity reaction caused by skin contact with specific allergens in sensitized individuals. It presents with erythema, itching, papules, vesicles, scaling, and chronic lichenification depending on the duration and severity of exposure. Accurate diagnosis is essential in occupational health and safety management because continued exposure to workplace allergens can result in chronic disease, reduced productivity, disability, and increased healthcare costs. A systematic investigation helps identify the causative allergen, distinguish ACD from other dermatoses, assess occupational exposure, and guide preventive measures. The first and most important investigation is a detailed clinical and occupational history. Information should be obtained regarding the onset of symptoms, duration, progression, distribution of lesions, seasonal variation, previous episodes, history of atopy, hobbies, cosmetic use, medications, and exposure to metals, rubber, chemicals, plants, dyes, preservatives, fragrances, and industrial materials. Occupational history should include the nature of work, duration of employment, specific job tasks, use of personal protective equipment, exposure to irritants and allergens, changes in workplace processes, and improvement of symptoms during weekends or vacations. Clinical examination provides valuable diagnostic clues. The morphology, distribution, symmetry, and severity of skin lesions should be documented. Lesions confined to exposed sites such as hands, face, eyelids, neck, or forearms may indicate occupational exposure. Examination should also identify secondary infection, lichenification, fissuring, pigmentary changes, nail involvement, or evidence of other dermatological conditions. Patch testing is the gold standard investigation for allergic contact dermatitis. It identifies the specific allergen responsible for delayed hypersensitivity reactions. Standardized allergens are applied to the patient's back using adhesive chambers and remain in place for approximately 48 hours. Readings are usually taken at 48 hours and again at 72–96 hours because delayed reactions may develop after removal of the patches. The interpretation includes grading of reactions as negative, doubtful, weak positive, strong positive, or extreme positive according to established criteria. Patch testing should be performed when dermatitis is inactive or minimally active and systemic corticosteroids or immunosuppressive therapy have been discontinued if possible. Identification of a positive allergen should always be correlated with clinical relevance and exposure history. Extended or specialized patch testing is indicated when standard series fail to identify the responsible allergen despite a strong clinical suspicion. Additional allergens based on occupation, cosmetics, medicaments, dental materials, textile dyes, plastics, glues, or industrial chemicals may be included. This improves diagnostic accuracy in workers exposed to specialized occupational allergens. Workplace material testing, also known as testing with the patient's own products, may be necessary when commercially available allergens do not include suspected substances. Samples of gloves, creams, adhesives, lubricants, cutting fluids, paints, resins, cleaning agents, or other workplace materials can be prepared in appropriate concentrations for patch testing. Care must be taken to avoid irritant reactions by using standardized dilution techniques. Repeat Open Application Test (ROAT) is useful when patch test results are doubtful or when confirming the clinical relevance of cosmetic or topical products. The suspected product is applied repeatedly to a small area of skin, usually the forearm, for several days to observe whether dermatitis develops. This test better simulates real-life exposure conditions. Use