Magnetic field plays a crucial role to understand most phenomena happening in the solar atmosphere. Sunspots, flares, prominences, coronal mass ejections are well known examples of its interaction with the solar plasma. To study the properties of this interaction, one needs to analyze the imprint it leaves in the radiation through the polarization induced in spectral lines, via the Zeeman and Hanle effects. Outside sunspots, the polarization degree of the emitted light is usually well below one part in one thousand, which requires sophisticated techniques to measure it accurately. To further complicate the situation, telescopes use mirrors and these introduce undesired polarization which is two or three orders of magnitude larger than that caused by the magnetic field of solar structures. For this reason, present telescopes doing polarimetry require an adequate modelling to correct the measured data from these spurious effects. In addition, most of the magnetic field interactions with the plasma take place at small scales. The best achievable angular resolution is mandatory to adequately study magnetic phenomena. The European solar Telescope (EST) has been defined to overcome these difficulties. Here, some aspects of the design are described.