Measurement of clinical attachment level has become a standard for judging clinical response in periodontal therapy. A number of probing methods and instruments have been developed in an attempt to address limitations in obtaining this measurement. First generation instruments include conventional periodontal probes; second generation probes utilize controlled forces; and third generation probes incorporate automated measurement, controlled forces, and computerized data capture. Various types of stents have been used and repeated measurement techniques have been proposed to reduce examiner error. Controlled force probes appear to have their greatest advantage in increasing inter-examiner repeatability. The use of measurement stents increases inter- and intra-examiner reliability. However, use of such stents may be limited to small sample studies of limited duration. Third generation instruments offer advantages in terms of automated measurement and data capture, increased resolution, and a more continuous measurement scale, but do not necessarily result in increased intra- or inter-examiner reliability. Examiner training and calibration are essential for any measurement instrument. Decisions for or against use of a particular instrument must be made on the basis of the needs of each clinical trial. At the present time, no commercially available instrument resolves all of the inherent limitations of clinical measurement of attachment.