A lot of knee injuries come under the broad title of ‘sports injuries’. The majority of these injuries will be a ligament sprain/tear or meniscal (footballer's cartilage).
Other knee pain can just start with no particular cause.
General knee rehabilitation will include exercises to improve the whole leg strength; quadriceps, hamstrings and hip muscles as well as exercises to improve balance.
For more information on knee pain read our Physio First document on the subject.