內丹術是道家一種重要的修鍊方法，現在一般視為道家氣功（道家多稱「煉氣術」）的一種。內丹術指以「人身是一小天地」的「天人合一」、「天人相應」思想為理論，進行性命的修鍊，以人身為鼎爐，修鍊「精、氣、神」等而達成強身健體、提高人體的生命功能、延長壽命、乃至成仙、長生不老之目的。
關於內丹修鍊的階次，各家方法有差，一般可分為築基、煉精化氣、鍊氣化神、煉神還虛幾個階段。元代陳致虛《金丹大要》卷四曰：「是皆不外神氣精三物，是以三物相感，順則成人，逆則生丹。何為順？一生二，二生三，三生萬物，故虛化神，神化氣，氣化精，精化形，形乃成人。何謂逆？萬物含三，三歸二，二歸一，知此道者怡神守形，養形煉精，積精化氣，鍊氣合神，煉神還虛，金丹乃成。」《金丹大要》還說：「求於冊者，當以《陰符》、《道德》為祖，《金碧》、《參同》次之。」
據傳道家文始派丹法修鍊下手即以最上一層”煉神還虛”做起。《文始經》曰：「能見精神而久生，能忘精神而超生。」《仙學真詮》釋之曰: 「蓋忘精神者，虛極靜篤，而精自然化氣，氣自然化神，神自然還虛，虛無大道之學也；見精神者，虛靜以為本，火符以為用，煉精成氣，鍊氣成神，煉神還虛，此以神御氣之術也。學虛無大道者，雖不著於精氣，然與道合真，神形俱妙，有無隱顯，變化莫測，其壽無量，是了性而自了命者也，舉上而兼下也；以神御氣，則著於精氣矣，然保毓元和，運行不息，沖和之至，薰蒸融液，亦能使形合於神，長生不死，乃了命而性因以存也，自下而做向上去也。此二端雖大小不同，而皆有益於人。」
《老子想爾注》中提出：「所以精者，道之彆氣也。」「萬物含道精。」「夫欲寶精，百行當修，萬善當著，調和五行。」《心印經》說：「精合其神，神合其氣，氣合其真，不得其真，皆是強名。」全真派《晉真人語錄》曰：「若人修行養命，先須積行累功。有功無行，道果難成。功行兩全，是謂真人。」「若要真功者，須是澄心定意，打疊精神，無動無作，真清真淨，抱元守一，存神固炁，乃真功也。」「若要真行，須要修行蘊德，濟貧拔苦，見人患難，常懷拯救之心，或化誘善人入道，修行所為之事，先人後己，與萬物無私，乃真行也。」張三丰以修人道為煉仙道的基礎，強調只要素行陰德，仁慈悲憫，忠孝信誠，全於人道，離仙道也就自然不遠了。他把道家的內煉思想同儒家學說合在一起，說：「人能修正身心，則真精、真神聚其中，大才、大德出其中。」
煉內丹方法還有「人元丹法」、「地元丹法」和「天元丹法」之別，「龍鳳丹法」與「龍虎丹法」之別，階次各有不同。據傳，龍鳳丹法源於上古的龍鳳紀。「龍鳳丹法」以《龍鳳金丹經》作為祖典而流傳。道家內丹術中豐富而精深的思想，對現代生命科學、人體科學及至哲學等學科的發展，有不可忽視的借鑒價值。
有學者認為精是氣和神緊密結合的產物，據此可以有從精中提取氣的煉精化氣的修行。氣就是實在的感覺慾望，神就是明晰的理性和自我意識。元氣是純粹的感受主體，元神是純粹的觀察主體。金丹是氣和神協調後返本。內丹術的修習步驟有四：煉體-煉精化氣、制氣-鍊氣化神、靜慮-煉神還虛、返觀-煉虛合道, 步驟之間可以重疊交錯。內丹術其實就是以氣為核心的精氣神之間的一個和諧化過程。
史書記載,內丹家多高壽,如《宋史》載：呂洞賓「年百餘歲而童顏，步履輕疾，頃刻數百里」；陳摶壽長118歲，張無夢99歲，張伯端96歲，石泰136歲，薛道光113歲，陳朴、劉海蟾、施肩吾、藍元道、陳楠、白玉蟾等內丹家都達到高齡。
近年來湖南長沙馬王堆出土之養生方法，敦煌所存之古代丹法，《道藏》中搜藏的內煉方法，均已廣泛印行，在中國學習者甚多。
The Daoist prescriptions or exercises to achieve immortality technique is the Taoism one kind important repairs 鍊 the method, now regards as the Taoism qigong (Taoism calls “refining technique” much) one kind generally. The Daoist prescriptions or exercises to achieve immortality technique refers to by “the person is a little world” “the beauty unites”, “the beauty to be corresponding” the thought is a theory, carries on the life to repair 鍊, take the person as the tripod with two handles stove, repairs 鍊 “the essence, the gas, the god” and so on to achieve strongs and healthy body, enhances human body’s life function, the extension life, and even becomes an immortal, goal of the immortality.
Repairs 鍊 the order about the Daoist prescriptions or exercises to achieve immortality, various method has the difference, generally may divide into builds the base, to build up the fine carburetion, 鍊 the gasified god, to build up the god also empty several stages. Yuan Dynasty Chen Zhixu “Jin Danda Wants” the volume four said: “is all nothing more than the spirited fine three thing, is by three phase feelings, along the adult, counter lives Dan. He Weishun? The life two, two lives three, three lives the myriad things, therefore the empty god, the god carburetion, the gasified essence, the essence changes from, the shape is an adult. What counter? The myriad things contain three, Sangui two, two normalizings, know this field of endeavor to gladden the heart defend the shape, raises the shape to build up the essence, accumulates the fine carburetion, 鍊 the gas gathers the god, builds up the god also empty, Jin Dannai becomes.””Jin Danda Wanted” added: “asks Yu Cezhe, when take “Cloudy Symbol”, “Morals” as the ancestor, “Gold and greens”, “the Senate with” is next.”
It is reported that Taoism Wen Shi sends the Dan law to repair 鍊 starts namely by on most ” Builds up god also empty ” Starting. “Wen Shijing” said: “can see the spirit, but for a long time lives, can forget the spirit to be reincarnated.””Immortal Study Real Annotation” releases it saying: “Ge Wang spirit, empty extremely static sincere, but the fine natural carburetion, is mad the naturalizing god, god naturally also empty, the nihility main road studies; Sees the spirit, empty calmly thinks the book, the fire symbol thinks uses, to build up fine becomes was mad, 鍊 is mad the god, builds up the god also empty, this controls one’s feelings the technique by the god. Studies the nihility main road, although in the essence, however gathers with the road really, the god shape is all wonderful, whether there is hidden obviously, unpredictable, its long-lived immeasurable, was the nature also, but from the life, has lifted but concurrently under; Controls one’s feelings by the god, then in the essence, however is guaranteeing the Yu Yuan and, does not move the rest, flushes and is the utmost, steams melts the fluid, can also cause the shape to be in harmony with the god, the immortality, was the life also because, but the nature saved, did from under to comes up. Although this two ends the size is different, but is all beneficial in the human.”
“Father Thought that the Note” proposed: “therefore the essence, 彆 is mad.”“the myriad things contain an essence.”“the husband wants the valuable essence, hundred trades repair, Wan Shandang, well distributed five lines.””Fortune-telling by the parts of Chinese characters after” said: “fine gathers its god, the god gathers its gas, the gas gathers it really, do not it really, all be the strong name.”All really sends “Jin Honorable person Collected sayings” said: “if the human leads a pious life raises the life, must accumulate a line of tired merit first. Active poor character, said that the fruit is hard to bring about. The merit line is satisfactory to both sides, is the saying honorable person.”“if the real merit, must be the clear heart makes decision, hits folds the spirit, not moves not does, really Islamic only, holds Yuan Shou one, saves the god solidly 炁, is the real merit.”“if really good, must lead a pious life yun Germany, helps the poor pulls out the pain, sees the person adversity, Chang Huai saves the heart, or tempts the good man to become a lay priest, to lead a pious life matter of the behavior, puts others before oneself, is selfless with the myriad things, is really good.”Zhang Sanfeng take repairs the humanity as to cultivate immortality a foundation, the emphasis, so long as the usual behavior good deeds done in secret, the benevolent grief, the loyal filial piety letter is honest, in the humanity, also naturally is not all far to immortal. He builds up Taoism’s in the thought to bring about a reconciliation with the Confucianist study in the same place, said: “the human can revise the body and mind, then really fine, the true God gathers in which, the great ability, Germany leave in which.”
Builds up the Daoist prescriptions or exercises to achieve immortality method also to have “person Yuan Danfa”, “Yuan Danfa” and “day Yuan Danfa” the difference, “the Longfeng Dan law” and “Long Hu the Dan law” the difference, the order has the difference respectively. It is reported that Longfeng Dan law origin in antiquity Longfeng discipline. “the Longfeng Dan law” spreads by “Longfeng Jin Danjing” as the ancestor standard. In Taoism Daoist prescriptions or exercises to achieve immortality technique rich and profound thought that to the modern life sciences, the human science until discipline and so on philosophy development, has noticeable profits from the value.
Some scholars thought that fine was mad with the god close union’s product, may have according to the above from the essence withdraws is mad builds up the fine carburetion leading a pious life. Was mad that is the solid feeling desire, the god is the defined rationality and the self-awareness. The vitality is the pure feeling main body, Yuan Shen is the pure observation main body. Jin Dan was mad after god coordinated returns to the book. The Daoist prescriptions or exercises to achieve immortality technique studies the step to have four: Builds up the body – to build up the fine carburetion, to vent one’s anger – 鍊 the gasified god, the static worry – to build up the god also empty, to return to the view – to build up empty gathers said that between the step may overlap interlock. The Daoist prescriptions or exercises to achieve immortality technique is actually take the gas as between a core vigor harmony process.
History book record, in alchemist multi-longevities, if “Song Shi” carries: Lu Dongbin “year hundred years old, but young face, step light illness, shortly several hundred miles”; Chen Tuan the long-lived long 118 years old, Zhang Wumeng 99 years old, Zhang Boduan 96 years old, the stone peaceful 136 years old, the Xue Daoguang 113 years old, Chen Po, the bangs toad, executes the shoulder I, blue Yuan Dao, Chen Nan, the white toad in the moon in and so on alchemist to achieve the advanced age.
In recent years Hunan Changsha Mawangdui unearthed keeping in good health method, Dunhuang saved the ancient times Dan law, “Daoist canon” searched for Tibet’s to build up the method, widely was printed and published by, in the Chinese learners very many.