U.S. patent law provides for liability for both direct and indirect infringement.
10.5.2.1 Direct infringement
Section 271(a) of the Patent Act imposes direct patent liability upon “whoever without authority makes, uses, offers to sell, or sells any patented invention, within the United States or imports into the United States any patented invention during the term of the patent therefor.” An accused product or process literally infringes a patent if it contains each and every limitation recited in a claim.
A defendant can also be held liable for nonliteral infringement where the accused product or process is close to the patented invention, but does not literally infringe. The doctrine of equivalents evolved in response to the concern that an “unscrupulous copyist” could avoid literal infringement of a patented invention by making insubstantial changes to the invention.107 Under the function-way-result test, an accused element is equivalent to a claim limitation “if it performs substantially the same function in substantially the same way to obtain the same result.”108 Under this test, a finding of equivalence requires that all three prongs be satisfied. The doctrine of equivalents determination is judged on the state of technology as of the time of the infringement, not (as in the case of means-plus-function claims) as of the time the patent issued.
The courts have limited the doctrine of equivalents in several ways. The all-elements rule provides that the test for equivalence under the doctrine of equivalents must be applied on an element-by-element (or limitation-by-limitation) basis. A finding of infringement therefore requires that the accused product or process contain each claim limitation or its equivalent.109 Moreover, the doctrine of equivalents is not available where the patentee has narrowed a claim element during prosecution unless (1) the equivalent was unforeseeable to a person having ordinary skill in the art at the time of the amendment, (2) the rationale for the amendment was no more than tangentially related to the equivalent at issue, or (3) another reason suggesting that the patentee could not reasonably be expected to have described the alleged equivalent.110 Furthermore, under the public dedication rule, a patentee may not invoke the doctrine of equivalents to recapture subject matter disclosed but not claimed in a patent.111
10.5.2.2 Indirect infringement
U.S. patent law also imposes liability upon those who actively induce or contribute to infringement by another person. Section 271(b) of the Patent Act provides that “[w]hoever actively induces infringement of a patent shall be liable as an infringer.” Induced infringement requires that the patentee prove that the defendant “actively and knowingly aid[ed] and abet[ted] another’s direct infringement.”112 The knowledge requirement can be established by showing actual or constructive knowledge of the patent113 or that the defendant acted with “willful blindness.”114 Under the doctrine of “willful blindness,” the inducer must have (1) subjectively believed that there was a high probability of infringement and (2) taken deliberate actions to avoid learning of that fact.115
Section 271(c) imposes liability under the following circumstances:
 Whoever offers to sell or sells within the United States or imports into the United States a component of a patented machine, manufacture, combination or composition, or a material or apparatus for use in practicing a patented process,  constituting a material part of the invention,  knowing the same to be especially made or especially adapted for use in an infringement of such patent,  and not a staple article or commodity of commerce suitable for substantial noninfringing use,  shall be liable as a contributory infringer.
The patentee must prove that the alleged contributory infringer had knowledge of the patent.116 Element  serves as an important defense, immunizing the sale of staple articles of commerce, that is, products that have substantial noninfringing uses. Thus, absent evidence of inducing conduct, sellers of non-patented goods are shielded from liability unless the good “has no commercial use except in connection with [… the] patented invention.”117