If you have been recovered from a knee injury and you are now returning to running, then you must then first try swimming and cycling. If you are okay with your knee, you may then graduate to running.

well, it depends on the case and the particular injury ... i've got about two injuries in one or the other knee each year and have for ... six years now ... my doctor has encouraged my running ... as well as my physical therapist ... one of the great things about physical therapy is that they can put you in a harness which will lighten the load of your body weight on your knees while recovering from an injury ... i've found that particularly helpful while dealing with an injury and still running during the season and attempting to compete ... that way, my muscles don't completely croak ... i ended up doing more damage swimming recovering from a knee injury than i would have done simply running ...

It depends on the injury i second this; I usually injure my knee while playing soccer but this does not necessary mean that i have to go and start cycling or swimming before starting to run and implicitly play soccer again. However if you come from a serious injury i think this might be a way to easier recover.