This report summarizes that portion of a research program on visual mid-air collision avoidance techniques which deals with the use of intentionally generated visible trails. The aim was to determine the effectiveness of such trails as conspicuity aids under daytime VFR conditions. Analytical review of the literature covers detection of various clouds, dusts, smokes and mists, their capability of indicating flight path, and their possible use with information coding techniques. Technical and operational considerations in using trails are also considered. The evidence provides both favorable and unfavorable indications of the merit of trails as aids for lessening the likelihood of collisions. In good weather and with crossing flight paths, detection range is increased. Technical operational disadvantages are: (a) high cost of installation and maintenance; (b) toxicity and other hazards to aircraft and communities; (c) severe weight requirements; and (d) unreliability of the visible signal. These disadvantages are great enough to preclude recommending a requirement that trails be generated for use in preventing mid-air collision.