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Motivation for PRO
There are two independent pieces of linguistic theory which motivated the existence of PRO. The Extended Projection Principle states that all clauses must have a subject, and the theta criterion states that every argument a verb can assign must be realized. These two theories initially appear to be violated by non-finite clauses, however introducing the empty category PRO addresses the concerns.

I persuaded John$i$ [PRO$i$ to read Al Gore's latest book]

In the example above, PRO serves as the subject of the non-finite clause [to read Al Gore's latest book], thereby satisfying the EPP-feature of T (occupied by the infinitival 'to' in the example). Since it is an object control sentence, PRO is coindexed with its antecedent 'John'.

Distribution of PRO
The distribution of PRO in infinitival clauses can be demonstrated by the following data. PRO has different distributions depending on whether it appears in an obligatory control or non-obligatory control context.

Distribution Under Obligatory Control
1 a. *It was expected PRO to shave himself. 1 b. *John thinks that it was expected PRO to shave himself. 1 c. *John’s campaign expects PRO to shave himself. (Hornstein 1999: 73)

1 a, b, c demonstrate that PRO's antecendent must be present, local and must c-command PRO.

1 d. John expects PRO to win and Bill does too. (= Bill win) (Hornstein 1999: 73)

1 d demonstrates that only the sloppy reading is allowed under VP ellipsis.

1 e. *John$i$ told Mary$j$ PRO$i+j$ to wash themselves/each other. (Hornstein 1999: 73)

1 e demonstrates that PRO may not have split antecedents.

Non-Obligatory Control Distribution
2 a. It was believed that PRO shaving was important. 2 b. John$i$ thinks that it is believed that PRO$i$ shaving himself is important. 2 c. Clinton’s$i$ campaign believes that PRO$i$ keeping his sex life under control is necessary for electoral success. (Hornstein 1999: 73)

In contrast to PRO under obligatory control, PRO under non-obligatory control may have an antecedent which is not local, does not c-command PRO, or which is not present at all.

2 d. John thinks that PRO getting his resume in order is crucial and Bill does too. (Hornstein 1999: 73)

2 d demonstrates that both the sloppy and strict readings are permitted. That is Bill may think that John having his resume in order is crucial, or that Bill thinks that having his own resume in order is crucial.

2 e. John$i$ told Mary$j$ [that PRO$i+j$ washing themselves/each other] would be fun. (Hornstein 1999: 73)

And 2 e demonstrates that PRO allows split antecedents under non-obligatory control.

A Movement Theory of Control
Norbert Hornstein (1999) has proposed that control verbs can be explained without resorting to PRO, and as such that PRO can be done away with entirely. This theory explains obligatory control with movement, and non-obligatory control with pro (little pro). This alternative theory of control was in part motivated by adherence to the minimalist program.

Working Assumptions
The Movement Theory of Control is predicated on the following principles.

(18) a. θ-roles are features on verbs. b. Greed is Englightened Self-Interest. c. A D/NP "receives" a θ-role by checking a θ-feature of a verbal/predicative phrase that it merges with. d. There is no upper bound on the number of θ-roles a chain can have. e. Sideward movement is permitted. (Hornstein 1999: 78)

Principle d is of particular importance as it allows the traces of a single D/NP to acquire more than one θ-role allowing it to simultaneously satisfy the θ-criterion for multiple positions, eg. the subject of the non-finite embedded clause and the subject of the matrix verb. Hornstein argues that there is insufficient empirical evidence that a chain must be restricted to a single θ-role and that allowing multiple θ-roles per chain is the null hypothesis.

Obligatory Control As Movement
These principles allow control verbs to be explained by movement and what had previously been interpreted as PRO can instead be interpreted as a residue equivalent to an NP-trace.

(19) a. John hopes to leave. b. [$IP$ John [{[sub|VP}} John [hopes [$IP$ John to [$VP$ John leave]]]]] (Hornstein 1999: 79)

This way the chain of Johns occupies the θ-roles of who is hoping and who is leaving, allowing the interpretation of obligatory control as movement and eliminating the need for PRO as a subject.

Non-Obligatory Control as pro
With the need of PRO eliminated under obligatory control Hornstein argues that it follows naturally to eliminate PRO altogether from the theory as it behaves equivalently to little pro under non-obligatory control. In non-obligatory control an overt embedded subject may be introduced or omitted, and omitting the embedded subject may result in an arbitrary reading. Additionally the overt subject may not be moved out of the embedded clause.

(43) a. It is believed that Bill’s/pro shaving is important. b. *Bill’s is believed that shaving is important. (44) a. It is impossible for Bill/pro to win at roulette. b. *Bill is impossible to win at roulette. (Hornstein 1999: 92)

Little pro may behave as a definite pronoun or indefinite (similar to the English one) which allows it to be distributed in the same way non-obligatory control PRO is observed to be distributed. This along with the observation that non-obligatory control arises when movement is not permitted places it in complementary distribution with obligatory control, as it is explained as movement. Since non-obligatory control occurs when movement is not permitted it may be treated as a 'elsewhere' case with little pro being inserted as a last resort measure to rescue the derivation if an overt subject is missing.

Criticism
Since the publication of this movement theory of control some data has been discussed which cannot be explained within the theory challenging the completeness of the movement theory of control.

Imoaka (2011) argues that scrambling out of a split control clause is incompatible with the movement theory of control as constructed in Japanese