A flush of technical developments and the resultant burgeoning research on channel molecules in biological membranes has, over the past two decades, led to clear operational distinctions between channel behavior and carrier behavior. These have evinced the tacit assumption that the two types of behavior are associated with distinctly different structural motifs (though certainly having some elements in common). Although a full survey of the physiological situation is hampered by low activation probabilities for most channels and by the small currents for most carrier systems (necessitating study of large ensembles of molecules), it is nevertheless clear that many intermediate types of behavior do exist, either in comparison of different channel and carrier molecules or in the responses of a given (class of) molecule at different times and in different membranes. Then in order really to understand the molecular mechanisms of transport, we must explicitly design experiments to isolate and quantify these unconventional modes of behavior.