In the legal context, evidence is something that is presented before a court of law in order to prove or disprove the facts of a case. It could be anything, such as a murder weapon, or a strand of hair, a document, a contract, a mangled vehicle, or statements of witnesses. So, there are different types of evidence, obtained from various sources. The form as well as source of the evidence may vary with different types. Different forms of evidence can be broadly classified into testimonial, physical, and documentary evidence. While physical evidence includes real objects, maps, models, diagrams, etc., documentary evidence means documents in writing, recordings, etc. Otherwise known as communicative evidence, testimonial evidence is the most basic form of evidence that involves statements made by witnesses under oath. In simple terms, testimonial evidence can be defined as sworn statements of witnesses made in open court. In its broad sense, this type of evidence includes oral and written statements made by witnesses under oath and the statements procured by the police from witnesses, victims, or suspects, during interrogation. Testimonial evidence is mainly about what the witnesses saw, heard, or know about the facts of the case. The most common example of testimonial evidence is the sworn statement of an eyewitness. An eyewitness testimony is considered as direct evidence. The statement of the witness is elicited through questions asked by the counsel of the party, who calls the witness. Once the direct examination is over, the other side can cross examine the witness. The opposing attorney can interfere during direct as well as cross examination, to disallow improper questions. Such objections can be raised when the questions are irrelevant, vague, inflammatory, or repetitive. In some cases, testimonial statements that were made out of the court (known as testimonial or non-testimonial hearsay evidence) are admitted into evidence, even if the declarant is unavailable for examination. This is mainly applicable in civil cases and not criminal cases, because criminal defendants have the right to be confronted with the witness against him (Sixth Amendment). A competent witness must be aware of the consequences of giving false testimony (perjury), which is considered as a felony according to the federal law, and is punishable with imprisonment up to five years. A lay witness must have personal knowledge that is relevant to the case. He must have sensed something that is relevant to the facts of the case. The witness must remember what he has sensed and should be able to communicate the same. In general, lay witnesses are not allowed to express their opinions and views. They have to communicate in the form of statements. However, this may become difficult in some circumstances. So, sometimes, witnesses are allowed to express their opinions; if such opinions are rational on the basis of what they have sensed, and are useful for understanding their testimony. In some cases, the evidence can be too technical or scientific in nature, in such a case experts are summoned as witnesses to testify with regard to such evidence. This helps the judge/jury to understand the evidence in a better manner and determine the facts in issue. Expert opinion is permitted only if his/her opinion is based on the facts or information perceived by that person; or if he can apply his expertise to interpret the data. In some cases, the court may appoint expert witnesses, on its own, or on a party's motion. Testimonial privileges include constitutional and non-constitutional privileges. Privilege against self-incrimination is provided by the Fifth Amendment of the Constitution. Others include communications privileges, to protect communication between doctor and patient, attorney and client, husband and wife, etc. A doctor cannot be compelled to disclose information about the health conditions and treatment plans of his patient, in a civil case. Similarly, a husband or wife cannot be compelled to testify against his/her spouse. However, waiver of this privilege is now permitted. Though testimonial evidence is considered more valuable than circumstantial evidence, it is not as reliable as physical evidence. Eyewitnesses are very vital, but even honest ones may make errors. It is really important to guard against such mistakes while collecting and handling such evidence.