This document discusses costing methods for HIV testing services. It aims to present key questions to interrogate cost evidence and support commissioning cost studies. The learning outcomes are to understand why HIV testing costs vary, recognize different cost definitions and analysis methods, and define what type of analyses are needed to address policy questions. It discusses financial vs. economic costs, full vs. incremental costs, project phases, perspectives, and inflation. Real differences in resource use can be due to scale, distribution model, population characteristics, and prevalence. The appropriate costing method depends on the question being asked - economic evaluation focuses on should we do it? while budget impact analysis addresses can we afford it?