The so called similarity principle -that structurally similar molecules share similar properties -is a key concept in drug discovery and molecular design. The main challenge in drug discovery is to find compounds with a combination of desirable properties such as absorption, distribution, metabolism, elimination and toxicity, safety and potency. Molecular optimization aims to address this challenge by exploiting the similarity principle, improving properties of molecules through small changes while still retaining or improving already desirable properties, for example retaining affinity against a drug target while improving aqueous solubility.