know what (you) are doing

to have the necessary knowledge or experience to do something.

Mountain climbing is a dangerous sport, so I hope he knows what he's doing.

• at the moment
• at the most
• at the outside
• at the point of
• at the point of something
• at the ready
• at the ripe old age of
• at the ripe old age of something
• at the top of agenda
• at the top of lungs
• at the top of someone's agenda
• at the top of the agenda
• at the top of the ladder
• at the top of voice
• at the top of your lungs
• at the top of your voice
• at the very least
• at this stage of the game
• at times
• attribute something to
• attribute something to someone
• attribute something to something
• attribute to
• attribute to someone
• attribute to something