Since the first experiment, made in 1990 [1], in which the emission of very efficient photoluminiscence by porous silicon could be showed, many works have been carried out in order to understand the reason why a porous material could present such a property, which is not present in bulk silicon. Bulk silicon is highly inefficient in emission of light and a drastic increase in efficiency can be obtained by a very simple treatment which converts bulk silicon in a porous sample. The importance of this new property is linked with the possibility of using porous silicon, instead of bulk silicon, as a base material for the development of optoelectronics. In this paper an attempt is made to give a general view on the theories developed in order to explain photo- and electroluminescence in porous silicon samples. The accent will be put onto considering the results of some works carried out in our laboratory, in relation to finding an explanation of the origin of visible electroluminescence in porous silicon layers.