This is the first chapter dealing with the construction of certain algorithms for synchronizing concurrent processes—the entry and exit protocols to protect critical sections. The effect of an entry protocol can intuitively be imagined in this way: A process closes access to a critical section as it passes through a door to it in order to reopen the door after leaving the critical section in the exit protocol. Therefore, the implementations of these protocols are called lock algorithmsLock algorithms and the corresponding data types locksLock. After the specification of locks, they are implemented using machine instructions—oriented to common processors—and these procedures are evaluated. Afterward, possibilities are presented, to implement entry and exit protocols for entering critical sections with elementary methods of sequential programming by accessing shared variables. Many of these solutions are “classical algorithms,” which have shaped research for years. However, a number of disadvantages and limitations—both conceptual and practical—also emerge.