Gravitational lenses are a consequence of the bending of light rays, which is one of the "classical" tests of Einsteins' General Relativity. This phenomenon appears when a large amount of mass, one or several galaxies, is located near the line-of-sight connecting the observer with a remote bright source, a quasar, far-off from both objects. In this case, the light traveling between the source and the observer can, due to the gravitational bending, follow two different paths, each of them at a different side of the central mass, which offers the possibility to observe two identical images of the source. If the emission from the quasar presents variability, the fluctuations in each image should be correlated: as the time light spends in traveling along different path varies from image to image, the light curves observed will follow the same pattern, but with an offset in time.After the discovery of the first gravitational lens, the double quasar 0957+561, in 1979, a great deal of effort to obtain the time delay between its two components has been made. This system consists of a pair of identical images, A and B (R magnitude 16, separation 6.1 arcsec), resulting from the bending by a cluster of galaxies of the light emitted from a quasar. Although a continuous monitoring of both components in several bands has been performed since the detection of 0957+561 there are, however, several controversies regarding the value of such time delay. It was only known that image B arrives with a delay larger than a year with respect to component A. The knowledge of the time delay between the images of a gravitationally lensed object is essential to determine the value of the Hubble constant, a quantity related with the age of the Universe and with cosmic distances, and to develop theoretical studies of the lensing object. Those are the reasons why an optical monitoring of the double quasar 0957+561 was set (see http://www.iac.es/lent) at the beginning of 1996 using the CCD camera of the 82 cm IAC80 telescope sited at the Instituto de Astrofisica de Canarias's Teide Observatory (Tenerife, Spain). However it still remains a great uncertainty in the value of the Hubble constant, and so it would be very useful to obtain the light curves as complete as possible. To do it, the contribution of the amateur astronomers is essential in order to obtain so many observations as possible (see http://www.iac.es/lent/amateur/light.hml for more details). With them, besides of obtaining the value of the Hubble constant, we could detect microlensing events, which give an estimation of the dark matter present both in the lensing galaxy and in our galaxy. In the same way, the detection of those phenomena would give information on the emitter, which is supposed to be an enormous black hole with and accretion disk around it.