Can you put a +1/+1 counter and a -1/-1 counter on the same creature and what happens?

Both +1/+1 counters and -1/-1 counters can exist on the same permanent, but the game has a built-in rule preventing them from coexisting for long. As a state-based action, whenever a permanent has both types of counters, the game removes one of each simultaneously, continuing until only one type remains (or none at all). This is defined in CR 122.6.

State-based actions are checked continuously and happen before any player gets priority, so the annihilation occurs almost instantly — players cannot respond to or prevent it. The process is automatic and not a triggered ability.

The practical result is that the counters effectively cancel each other out. If a 2/2 creature has two +1/+1 counters and you somehow add two -1/-1 counters, all four counters are removed, leaving a 2/2 again. If the numbers are unequal, only the excess of the larger group remains.

Example: Your Devoted Druid (a common -1/-1 counter user) has two +1/+1 counters on it. You activate its ability to put a -1/-1 counter on it. State-based actions fire: one +1/+1 counter and one -1/-1 counter are removed simultaneously, leaving Devoted Druid with one +1/+1 counter and no -1/-1 counters.

This rule is important in deck-building and gameplay because it means +1/+1 and -1/-1 counter synergies rarely interact additively — they simply erase each other, which can be exploited deliberately in some strategies.