In this review we attempt to present a critical assessment of current knowledge about physical and chemical factors that affect feeding by bloodsucking insects, emphasizing events that occur after tarsal contact. Such a review is timely because of an increasing number of reports on phagostimulatory substances in natural and artificial diets based on experiments in which the complexity and variability of the feeding response often may be overlooked. The reader is referred to other reviews on the behavior, physiology, and mor phology of blood-feeding insects concerning the following topics: general orientation and mouthpart morphology (3); phagostimulants (32, 38, 39, 75); regulation of feeding (6, 47, 48); evolution of blood feeding, general behavior and physiology, and orientation to host (23, 53); host-ectoparasite interactions (82, 105); structure of chemoreceptors (15, 97); nutrition (57); symbionts ( 14); digestion (49); feeding habits (24, 25, 26, 37); saliva of Hemiptera (79); and metabolism of blood meal ( 17). The habit of blood feeding has arisen independently in a wide range of insects, sometimes within the same order (3). Differences in feeding mechanisms, sensory receptors, and host characteristics result in differences in the factors affecting feed ing and in their interplay; however, there are also features common to the process. Dethier (23) suggests that feeding in mosquitoes is the culmination of a stepwise series of stimUlus-response events, as shown in plant-feeding insects. This probably applies to blood feeders generally. The events include detecting the host, alighting 00 it, probing (active movement of the mouthparts towards the surface), piercing or penetrating (insertion of mouthparts into the surface), locating blood (either in a blood vessel or from a hemorrhage), taking up blood into part or parts of the gut, and ceasing feeding. The earlier events may be missing if the insect remains in contact with the host between meals. Factors affecting feeding may determine individual events in the sequence or may modify parts or all of it. The process may