• Sequential programs use shared variables for global data structures, but they can be written without them.

• However, concurrent programs absolutely depend on the use of shared components, because the only way processes can work together is to communicate:
• and the only way to communicate is that one process writes into something that the other process reads
• that something can be a shared variable (in shared-variable programming) or it can be a shared communication channel (in distributed programming)

• Mutual exclusion occurs whenever two processes need to take turns accessing shared objects (e.g., a shared client account record in database), condition synchronization occurs whenever one process needs to wait for another (e.g.,  a server waiting for a client)

• Shared-variable programs are most commonly executed on shared-memory machines, because every variable can be accessed directly by every processor, however they can be used on distributed-memory machines too (as a distributed shared memory)