In this chapter, we first revisit the basic concepts of database transactions, and discuss how these concepts are achieved in practical systems. Next, we briefly go through the architecture of transaction processing systems in the centralized and the distributed environments. This chapter we have reviewed the basic concepts of database systems and database transactions, and discussed the architecture of transaction processing systems in distributed environments. We will shift our focus to transactions and transaction processing in mobile environments, which possess some unique characteristics such as the mobility of mobile computing hosts, the limitations of wireless communications and the resource constraints of mobile computing devices.