What are some possible reasons that my knees crack when walking up stairs?

Dont worry: The most common and likely cause of cracking in front of the knee is an irritated or early wear of the knee cap cartilage or the surface cartilage of the thigh bone that it rubs. Other causes could include a plica, patellar tendonitis etc. For starters avoid stairs, knee extension machines, deep knee squats, lunges etc. Try therapy, antiinflammatories, ice and sometimes a knee sleeve. See ortho.