Pharmaceutically active compounds are complex molecules with different functionalities, physicochemical and biological properties. They are developed and used because of their more or less specific biological activity and are most notably characterised by their ionic nature. Their molecular weights range typically from 300 to 1 000. Under environmental conditions molecules can be neutral, cationic, anionic, or zwitterionic. They also often have basic or acidic functionalities. Pharmaceuticals can be classified according to their effects, but also “crosswise” according to their chemical structure. Normally, pharmaceuticals and disinfectants are classified according to their therapeutical purpose (e.g. antibiotics, analgesics, antineoplastics, anti-inflammatory substances, antibiotics, antihistaminic agents, contrast media, etc.). Classification according to chemical structure is used mainly for the sub-groups of the active substances, e.g. within a group of antibiotics such as β-lactams, cephalosporins, penicillins or quinolones. In such cases, some of the compounds can be treated as groups and one or the other compound can be used as a general example for this group. A closely related chemical structure may be accompanied by an identical or at least a similar mode of action (e.g. antibiotics). However, as the example of antineoplastics shows, it might also be very different: alkylating, antimetabolic, mitosis inhibiting or intercalating substances belong to different classes of chemicals. In other words, compared to most bulk chemicals, pharmaceutically active compounds are often complex molecules with special properties e.g. dependence of log K ow, on pH (see Chap. 2).