Apologetics (from Ancient Greek ἀπολογία (apología) 'speech in defense') is the religious discipline of defending religious doctrines through systematic argumentation and discourse. Early Christian writers (c. 120–220) who defended their beliefs against critics and recommended their faith to outsiders were called Christian apologists. In 21st-century usage, apologetics is often identified with debates over religion and theology.

The term apologetics derives from the Ancient Greek word apologia (ἀπολογία). In the Classical Greek legal system, the prosecution delivered the kategoria (κατηγορία), the accusation or charge, and the defendant replied with an apologia, the defence. The apologia was a formal speech or explanation to reply to and rebut the charges. A famous example is Socrates' Apologia defense, as chronicled in Plato's Apology.

In the Koine Greek of the New Testament, the Apostle Paul employs the term apologia in his trial speech to Festus and Agrippa when he says "I make my defense" in Acts 26:2. A cognate form appears in Paul's Letter to the Philippians as he is "defending the gospel" in Philippians 1:7, and in "giving an answer" in 1 Peter 3:15.

Although the term apologetics has Western, primarily Christian origins and is most frequently associated with the defense of Christianity, the term is sometimes used referring to the defense of any religion in formal debate involving religion.

Many apologetic books have been written in defence of the history or teachings of the Baháʼí Faith. The religion's founders wrote several books presenting proofs of their religion; among them are the Báb's Seven Proofs and Bahá'u'lláh's Kitáb-i-Íqán. Later Baháʼí authors wrote prominent apologetic texts, such as Mírzá Abu'l-Fadl's The Brilliant Proof and Udo Schaefer et al.'s Making the Crooked Straight.

One of the earliest Buddhist apologetic texts is The Questions of King Milinda, which deals with the Buddhist metaphysics such as the "no-self" nature of the individual and characteristics such as wisdom, perception, volition, feeling, consciousness and the soul. In the Meiji Era (1868-1912), encounters between Buddhists and Christians in Japan as a result of increasing contact between Japan and other nations may have prompted the formation of Japanese New Buddhism, including the apologetic Shin Bukkyō (新仏教) magazine. In recent times, A. L. De Silva, an Australian convert to Buddhism, has written a book, Beyond Belief, providing Buddhist apologetic responses and a critique of Christian Fundamentalist doctrine. Gunapala Dharmasiri wrote an apologetic critique of the Christian concept of God from a Theravadin Buddhist perspective.

Christian apologetics combines Christian theology, natural theology, and philosophy in an attempt to present a rational basis for the Christian faith, to defend the faith against objections and misrepresentation, and to show that the Christian doctrine is the only world-view that is faultless and consistent with all fundamental knowledge and questions.

Christian apologetics has taken many forms over the centuries. In the Roman Empire, Christians were severely persecuted, and many charges were brought against them. Examples in the Bible include the Apostle Paul's address to the Athenians in the Areopagus (Acts 17: 22-34). J. David Cassel gives several examples: Tacitus wrote that Nero fabricated charges that Christians started the burning of Rome. Other charges included cannibalism (due to a literal interpretation of the Eucharist) and incest (due to early Christians' practice of addressing each other as "brother" and "sister"). Paul the Apostle, Justin Martyr, Irenaeus, and others often defended Christianity against charges that were brought to justify persecution.