The better return on investment that horizontal drilling techniques provide has prompted an increase of this activity worldwide. Initially, it was felt little or no data were required to evaluate the reservoirs produced through horizontal boreholes because early completion techniques consisted of simple openhole completions or placement of a slotted liner along the lateral borehole. However, because of productivity anomalies, a need for thorough evaluation techniques soon became apparent. This paper is a review of the latest techniques used to evaluate horizontal wells. Methods of instrument conveyance, formation evaluation techniques, and measurement-while-drilling applications are included.