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Circumstantial evidence is evidence that relies on an inference to connect it to a conclusion of fact—like a fingerprint at the scene of a crime. By contrast, direct evidence supports the truth of an assertion directly—i.e., without need for any additional evidence or inference.
On its own, it is the nature of circumstantial evidence for more than one explanation to still be possible. Inference from one piece of circumstantial evidence may not guarantee accuracy. Circumstantial evidence usually accumulates into a collection, so that the pieces then become corroborating evidence. Together, they may more strongly support one particular inference over another. An explanation involving circumstantial evidence becomes more valid as proof of a fact when the alternative explanations have been ruled out.
Circumstantial evidence allows a trier of fact to deduce a fact exists. In criminal law, the inference is made by the trier of facts in order to support the truth of assertion (of guilt or absence of guilt).
Testimony can be direct evidence or it can be circumstantial. If the witness claims they saw the crime take place, this is considered direct evidence. For instance, a witness saying that the defendant stabbed the victim is direct evidence. By contrast, a witness who says that she saw the defendant enter a house, that she heard screaming, and that she saw the defendant leave with a bloody knife gives circumstantial evidence. It is the necessity for inference, and not the obviousness of a conclusion, that determines whether or not evidence is circumstantial.
Forensic evidence supplied by an expert witness is usually circumstantial evidence. A forensic scientist who testifies that ballistics proves the defendant’s firearm killed the victim provides circumstantial evidence from which the defendant’s guilt can be inferred. (Note that an inference of guilt could be incorrect if the person who actually fired the weapon was somebody else.)
On the other hand, the additional circumstantial evidence of the defendant's fingerprint on the trigger would dovetail with this piece to provide corroborating evidence.
Circumstantial evidence is especially important in civil and criminal cases where direct evidence is lacking.
Circumstantial evidence is used in civil courts to establish or refute liability. It is usually the most common form of evidence, for example in product liability cases and road traffic accidents. Forensic analysis of skid marks can frequently allow a reconstruction of the accident. By measuring the length of such marks and using dynamic analysis of the car and road conditions at the time of the accident, it may be found that a driver underestimated his or her speed. Forensic science and forensic engineering are common as much in civil cases as in criminal.
With obvious exceptions (immature, incompetent, or mentally ill individuals), most criminals try to avoid generating direct evidence. Hence the prosecution usually must resort to circumstantial evidence to prove the mens rea levels of "purposely" or "knowingly." The same goes for tortfeasors in tort law, if one needs to prove a high level of mens rea to obtain punitive damages.
One example of circumstantial evidence is the behavior of a person around the time of an alleged offense. If someone was charged with theft of money and was then seen in a shopping spree purchasing expensive items, the shopping spree might be circumstantial evidence of the individual's guilt.
Other examples of circumstantial evidence are fingerprint, blood analysis or DNA analysis of the evidence found at the scene of a crime. These types of evidence may strongly point to a certain conclusion when taken into consideration with other facts—but if not directly witnessed by someone when the crime was committed, they are still considered circumstantial. However, when proved by expert witnesses, they are usually sufficient to decide a case, especially in the absence of any direct evidence. Owing to developments in forensic methods, old undecided cases (or cold cases) are frequently resolved.
A popular misconception is that circumstantial evidence is less valid or less important than direct evidence. This is only partly true: direct evidence is popularly, but mistakenly, considered more powerful. Many successful criminal prosecutions rely largely or entirely on circumstantial evidence, and civil charges are frequently based on circumstantial or indirect evidence. Much of the evidence against convicted American bomber Timothy McVeigh was circumstantial, for example. Speaking about McVeigh's trial, University of Michigan law professor Robert Precht said, "Circumstantial evidence can be, and often is much more powerful than direct evidence".  The 2004 murder trial of Scott Peterson was another high-profile conviction based heavily on circumstantial evidence.
Indeed, the common metaphor for the strongest possible evidence in any case—the "smoking gun"—is an example of proof based on circumstantial evidence. Similarly, fingerprint evidence, videotapes, sound recordings, photographs, and many other examples of physical evidence that support the drawing of an inference, i.e., circumstantial evidence, are considered very strong possible evidence.
In practice, circumstantial evidence can have an advantage over direct evidence in that it can come from multiple sources that check and reinforce each other. Eyewitness testimony can be inaccurate at times, and many persons have been convicted on the basis of perjured or otherwise mistaken testimony. Thus, strong circumstantial evidence can provide a more reliable basis for a verdict. Circumstantial evidence normally requires a witness, such as the police officer who found the evidence, or an expert who examined it, to lay the foundation for its admission. This witness, sometimes known as the sponsor or the authenticating witness, is giving direct (eye-witness) testimony, and could present credibility problems in the same way that any eye witness does.
However, there is sometimes more than one logical conclusion inferable from the same set of circumstances. In cases where one conclusion implies a defendant's guilt and another his innocence, the "benefit of the doubt" principle would apply. Indeed, if the circumstantial evidence suggests a possibility of innocence, the prosecution has the burden of disproving that possibility.