The possibilities offered by low-temperature scanning tunnelling microscopy to perform controlled manipulation of single atoms and small molecules and to build structures at the atomic scale have been extended recently to the manipulation of specially designed organic molecules. In this article, some recent applications of the manipulation techniques to large complex molecules are reviewed. Single porphyrin-based molecules deposited on copper substrates have been manipulated by means of their interaction with the microscope tip. Manipulation of the molecules on Cu(211) offers the possibility to modify selective their internal conformation, realizing the principle of a conformational molecular switch. In a constant height mode of manipulation, the tunnelling current recorded during the process contains detailed information on the internal mechanics of ihe molecule when guided by the microscope tip.