A SIM, Subscriber Identity Module, is the removable circuit board found in a modern cellular phone. It carries the network identity information and is a type of smart card which can also be found on payment cards (EMV), ID cards and so on. A smart card is basically a small computer, providing a safe and controlled execution environment. Historically smart card software was very hardware dependent and mostly developed by the manufacturers themselves. With the introduction of the open Java Card standard created by Sun Microsystems (Oracle) this was meant to change. However, information still remains scattered and is hard to obtain. This paper is meant to serve both as an introduction to the field and also as a good foundation for future studies. It begins with a theoretical discussion about smart card hardware and software architectures, network standards in the context of SIM cards, typical applications, coming trends and technologies and ends off with an overview of the Java Card standard. The following section discusses the supplied example SIM card application coupled with an introduction how to use the Gemalto Developer Suite for application development and testing. The paper ends with an extensive appendix section going in depth about some of the more important subjects.