Instant, {G/U} (1)
Common
Move any number of +1/+1 counters from target creature onto another target creature with the same controller.
"It's all right if you change your mind. There's always a chance we can rearrange parts later."
—Zija, Simic mutationist
Illus. Scott Chou

1/24/2013: To move a counter from one creature to another, the counter is removed from the first creature and placed on the second. Any abilities that care about a counter being removed or placed on a creature will apply.
1/24/2013: You decide how many counters to move when Bioshift resolves.
1/24/2013: If one of the two creatures is an illegal target when Bioshift tries to resolve, or if the creatures are controlled by different players at that time, no counters will move.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Extended (Type 1.X)
Legal in Standard (Type 2)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern