The followers of Islam are obligated to understand the principles of Islamic economic system, and what is permissible and prohibited under Islamic law. Islam enjoins the faithful to consume what is lawful (ḥalāl) and prohibits them from consuming what is illicit (ḥarām), impure and forbidden. In this context, it becomes inevitable to understand the licit and illicit transactions in Islamic economics, specifically for those people who sell in the marketplaces of the Muslims. This chapter begins with an introduction to the basic principles for licit and illicit transactions and then discusses lawful and un-lawful businesses in Islam. In this chapter, many contemporary issues – like donation and selling of human body organs, sale and purchase of gambling products, transaction of interest-based commodities, contract ambiguities, sale and buy-back (bay’ al-‘īna), sale of items that are not in one’s possession, sale on sale, animals that feed on impurities (al-jallāla), lotteries and Bitcoin – are also discussed in a very comprehensive manner.