Short answer
Clothes that cover the skin but are so tight that they show the shape of the awrah are not allowed in front of others, for men or women. In the Hanafi school a prayer in them is still valid, though disliked. A fitted shirt or trousers that do not cling to the awrah are fine for men.
If clothing clings so that the shape of the awrah can be seen, wearing it before others is impermissible and looking at that shape is not allowed. The prayer is valid because the skin is covered, but it is makruh. Thin clothing that shows the colour of the skin does not count as covering at all.
The four schools agree on this.
Explanation
Covering the awrah has two parts: the skin must not show, and the shape must not be on display. The Prophet ﷺ described women of the future who would be "clothed yet naked", which the scholars explain as clothing that is thin or tight enough to reveal what it should hide. The same principle applies to men within their awrah. The four schools agree on this.
For men. Skinny jeans, compression shorts, cycling shorts and gym leggings worn on their own can show the shape of the private parts and thighs. Wear something looser over them, like shorts that reach below the knee or a longer top. A fitted shirt is fine, because the chest is not part of a man's awrah.
For women. The awrah in front of non-mahram men covers much more, so loose clothing matters more. Details are on the hijab page and the awrah guide.
See-through clothes. If the colour of the skin shows through, the awrah is not covered, and a prayer in such clothes is invalid if the see-through part is the awrah. Check thin white trousers and light fabrics against the light.
In prayer. A Hanafi prayer in tight but opaque clothes counts, but it is better to pray in loose clothing. Allah tells us to take our adornment at every mosque, which includes dressing with dignity for prayer.
The evidence
Quran, Surah Al-A'raaf 7:26
يَٰبَنِىٓ ءَادَمَ قَدْ أَنزَلْنَا عَلَيْكُمْ لِبَاسًۭا يُوَٰرِى سَوْءَٰتِكُمْ وَرِيشًۭا ۖ وَلِبَاسُ ٱلتَّقْوَىٰ ذَٰلِكَ خَيْرٌۭ ۚ ذَٰلِكَ مِنْ ءَايَٰتِ ٱللَّهِ لَعَلَّهُمْ يَذَّكَّرُونَ ﴿٢٦﴾
O children of Adam, We have bestowed upon you clothing to conceal your private parts and as adornment. But the clothing of righteousness - that is best. That is from the signs of Allah that perhaps they will remember.
Clothing should conceal, not display.
Quran, Surah Al-A'raaf 7:31
۞ يَٰبَنِىٓ ءَادَمَ خُذُوا۟ زِينَتَكُمْ عِندَ كُلِّ مَسْجِدٍۢ وَكُلُوا۟ وَٱشْرَبُوا۟ وَلَا تُسْرِفُوٓا۟ ۚ إِنَّهُۥ لَا يُحِبُّ ٱلْمُسْرِفِينَ ﴿٣١﴾
O children of Adam, take your adornment at every masjid, and eat and drink, but be not excessive. Indeed, He likes not those who commit excess.
Dress well and modestly for prayer.
Sahih Muslim 2128
Sahih... ونساء كاسيات عاريات مميلات مائلات ...
"And women who are clothed yet naked, who lead others astray and are led astray."
Clothing that reveals the body does not fulfil the purpose of covering.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Salah, chapter on the conditions of prayer, covering the awrah. A tight garment that is opaque does not invalidate prayer, while one that shows the skin does.
- al-Fatawa al-Hindiyya, Kitab al-Karahiyya, chapter on looking and touching. Looking at the shape of the awrah through a clinging garment.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.