Spectroscopic and photometric variabilities are well established phenomena for many massive stars. The origin of these variabilities is not yet fully understood. Radial and non-radial pulsations have been suggested as one of the responsible candidates for inducing variabilities. In this context, I will discuss about instabilities and their consequences in models of massive zero age main sequence as well as post-main sequence stars. These instabilities may lead to finite amplitude pulsations, direct mass-loss from the stellar surface or even disruption of a whole star in extreme cases. I will also present a comparison of theoretical predictions with the recent observations which enhances our present understanding of variabilities and mass-loss mechanism in massive stars.