The Assistance of Counsel Clause is a part of the Sixth Amendment to the United States Constitution.  It says: "In all criminal prosecutions, the accused shall enjoy the right...to have the Assistance of Counsel for his defense."  This means that anyone being accused of a crime has the right to have a lawyer help defend them.  ("Counsel" is a legal word for "lawyer.")

Meaning

Since the Constitution was written, many questions have come up about what exactly the Assistance of Counsel Clause means.  The Supreme Court of the United States has made many decisions to answer these questions.  The Court has decided that Americans have five different rights under the Assistance of Counsel Clause:
 The right to a lawyer who does not have any conflicts of interest
 The right to choose what lawyer they want
 The right to have a free lawyer assigned to them if they cannot pay for a lawyer
 The right to a lawyer who is good enough at their job to be effective
 The right to defend themselves, and act as their own lawyer

When the right to counsel starts
The Supreme Court case Brewer v. Williams, , answered the question of when a person's right to a lawyer begins.  In this decision, the Court said a person has the right to have a lawyer's help "at or after":
 They are charged with a crime
 They have a court hearing
 They are indicted or arraigned (formally charged with a crime)

After any of these things happen, a person has the right to a lawyer whenever they are interrogated.

In the case Escobedo v. Illinois, , the Court also decided that suspects have the right to a lawyer when they are being interrogated by the police, even if they have not been charged with a crime yet.

People who go in front of grand juries do not have the right to have a lawyer with them.  This is because the Supreme Court does not think of grand juries as "criminal prosecutions."  (A grand jury cannot find someone guilty of a crime or punish anyone; they can only decide that a person should be charged with a crime.)

Conflict-free counsel
Every defendant has the right to a lawyer without a conflict of interest.  Here is an example of a conflict of interest: a lawyer is defending two different people.  A client named Smith wants to blame his crime on another client named Jones.  A lawyer is supposed to do what is best for their clients.  Here, the lawyer cannot do that for both clients.  If the lawyer blames Smith's crime on Jones in court, he is hurting Jones.  If he does not blame the crime on Jones, he is refusing to defend Smith the way Smith wants.  This is not fair to either client.  Also, the lawyer cannot do the best possible job for both clients at the same time.  

To make sure they get a fair trial, every defendant has the right to a lawyer without a conflict of interest.  It does not matter whether the defendant is paying the lawyer, or the lawyer was assigned for free.  If a lawyer has a conflict of interest, and it hurts their defense of their client, an appeals court will automatically cancel the defendant's conviction.

Right to counsel of choice
Defendants being charged with a crime have the right to be represented by the lawyer they want.  However:
 That lawyer must be licensed to practice law in the jurisdiction where the defendant is being tried
 The lawyer must be willing to be the defendant's lawyer (either for free or for pay)
 The lawyer cannot have a conflict of interest

If a court violates a defendant's right to counsel of their choice, and the defendant appeals, the appeals court will automatically cancel the defendant's conviction.

Appointment of counsel

Defendants have the right to have a lawyer assigned (appointed) to them for free, if:
 They are being charged with a capital crime
 They are too poor to pay for a lawyer, and they are being charged with:
 A felony
 Any other crime that they could be sent to prison for

Ineffective assistance of counsel

The Supreme Court has ruled that Americans have the right to an effective lawyer.  For example, if a defendant is charged with a crime, a court cannot appoint just any lawyer.  The defendant must be given a lawyer who can try a criminal case, and can defend their client effectively.

The Supreme Court explained what "ineffective assistance of counsel" means in a case called Strickland v. Washington (1984).  In this case, a defendant was convicted.  He appealed, saying that his lawyer did not do a good enough job defending him.  The Court ruled that he could only have a new trial if he proved:
 His lawyer did not do an adequate job of defending him; and
 If the lawyer did a better job, the defendant may not have been convicted.

If a defendant can prove these things to an appeals court, they have proved that their lawyer was ineffective (not effective).  This means they have proved their Sixth Amendment rights were violated.  They will then get a new trial with a different lawyer.

Self-representation
In most cases, a defendant being charged with a crime has the right to defend himself.  This is called pro se representation.  It means the defendant has given up their right to have a lawyer.  The defendant has decided to act as their own lawyer.

However, a court can rule that the defendant will not be able to give himself a good enough defense.  If that happens, the court can rule that the defendant has to have a lawyer. 

Americans do not have the right to defend themselves in appeals courts.  In these courts, a defendant must have a lawyer.

Related pages
 Sixth Amendment to the United States Constitution
 Escobedo v. Illinois (created the right to a lawyer during interrogation)
 Gideon v. Wainwright (created the right to a free lawyer for poor defendants)
 Ineffective assistance of counsel

References

United States Constitution