Criticism of communism can be divided into two broad categories: those concerning themselves with the practical aspects of 20th century Communist states, and those concerning themselves with communist principles and theory. Authors who are critical of communism are typically opposed to both communist principles and historical policies, though they may focus exclusively on one or the other aspect in their writing.
Some anti-communists argue that communist theory is directly responsible for the problems of 20th century "communism", while others separate theory from practice while maintaining that they are both flawed in their own way. Likewise, some communists defend both theory and practice, while others argue that historical practice diverged from communist principles to a greater or lesser degree.
One may agree with communist principles but disagree with many policies adopted by Communist states (this is quite common among Trotskyists), or, more rarely, agree with policies adopted by Communist states but disagree with communist principles.
The categories of criticisms mentioned above are discussed in separate articles:Criticisms of communist party rule is an article dealing with criticisms of the practical policies implemented by 20th century governments claiming to follow the ideology of Marxism-Leninism (these are usually called "Communist states" in the West).
Criticisms of Marxism is an article dealing with criticisms of Marxist theory itself.