The analysis of relevant data to develop a market value opinion requires two important steps in the valuation process before the applicable approaches to value are applied. Market/marketability analisys begins the process of narrowing the focus from a broader macro view to data that is especially pertinent to the appraised property. Highest and Best Use is the overriding to be applied in the analysis and ultimate selection of comparable data used in real estate valuation. In all valuation assignments, opinions of value are based on use. Highest and Best Use relies on that analisys to identify the most profitable, competitive use to which the subject property can be put. Highest and Best Use is often identified as the key concept supporting real estate use and value decisions. This paper addresses this ambiguity and identifies the theoretical premises of Highest and Best Use as employed in the various land use disciplines. There are many highest and best use problems associated with real estate appraisels today.