FATMAP difficulty grade
The Jungfrau is a famous and beautiful mountain with no very easy route to its summit.
As described here this is an out and back route from the Jungfraujoch railway.
However this is only suitable for very acclimatised and very fit people.
Even if you tick both those boxes it is unsuitable in particularly hot weather, due to the sunny nature of the route, as the descent may prove dangerous.
If in doubt stay in the Mönchsjoch Hut and make an alpine start.
It is also a very conditions-dependent route, varying in difficulty and also having different variations, so seek local advice on current conditions. From the Jungfraujoch railway exit onto the glacier and take a long and initially gently descending traverse onto and across the Jungfrau Glacier at about 3300m, passing below an obvious band of large crevasses.
You are aiming for the bottom of the rocky spur that projects L onto the glacier from the Rottalhorn.
Go easily up rocks past a rain gauge, then more steeply, passing some pegs. Avoid the steepest section that follows by traversing L to gain the crest.
This whole section can be avoided when snowy, by traversing around the base of the spur and climbing directly up a 30 degree snow slope.
From the top of the spur follow it easily in the direction of the Rottalhorn, which you pass beneath.
You are aiming for the Rottalsattel, which may involve negotiating a tricky bergschrund.
From the col climb up a short distance then traverse L to gaining the rocky ridge, which is followed at its top rightwards to the summit.
The ridge has assorted bits of metalwork every 25-50 meters to protect it in both directions.
The ridge to its R is normally icy in the summer, but in spring or very snowy conditions it will be preferable in ascent and descent, when it has good steps in it.
Descent is the same in reverse, but in exceptional conditions it is possible to descent directly from the Rottalsattel, saving time, but this is exposed to serac fall, so not recommended in ascent.