The parameters for signal intensity in magnetic resonance imaging (MRI) are classified and shortly described. Proton density, T1 and T2 are discussed as tissue parameters to be influenced in different ways by different types of contrast agents. Conclusions for the development of contrast agents for MRI are drawn. The influence of the concentration of paramagnetic substances on signal intensity is shortly described. The good tolerance of Gd-DTPA as a contrast agent for MRI is reported. Its favorable use is shown in three examples of brain tumors.