The outbursts of classical novae are among the strongest explosions in the Universe. The multifrequency observations are contributing to understanding the process of explosions and of the long term evolution. In this review, I discuss the observations of novae over the electromagnetic spectrum, focusing on the morphology of the decline light curves, the spectroscopic investigations, the long term evolution, the recurrent novae, the gamma ray emission in novae, extragalactic novae and the gravitational emission of novae.