The advent of infrared and microwave technologies opened up a new era in the study of mass loss from red giants. Physical structure (temperature, density and velocity distributions) of the circumstellar (CS) envelopes of red giants can be determined through both line and continuum observations in these new wavebands (cf Kwok 1980a). In this paper, we compare the spectral characteristics of red-giant envelopes and planetary nebulae (PN) and discuss the possible spectral evolution from one to the other.