P-glycoproteins are an interesting group of proteins with positive effect since it eliminates harmful substances, but it also may eliminate drugs and reduce the therapy efficiency. This thesis explains what P-glycoproteins are and what their role is. P-glycoprotein inhibitors and how they work is also explained. This work describes first, second and third generation of inhibitors and explains why they have more advantages than first and second generation of inhibitors. Also, some of the representatives of the third generation of inhibitors are listed and some of their characteristics are described. Furthermore, multiple drug resistance and why it is an obstacle in successful therapy against some diseases are described. Also, the link between P-glycoprotein and multiple drug resistance has been elucidated. This thesis explains the role of MDR1 (multidrug resistance) genes in drug resistance, their expression and modulation. The interaction between active substances in plants and P-glycoproteins, their cytotoxicity and the methods by which medical chemistry detects P-glycoprotein inhibitors are described. The plants whose extract are used and explanation why these extracts are suitable in the treatment of cancer are discussed. Furthermore, the ways in which medical chemistry detects P-glycoprotein inhibitors are described, some novel compounds are listed, and also the explanation of mechanism of P-glycoprotein inhibition is given.