More than 200 people have been killed in the Gaza Strip after Israel dropped lots of bombs there, local medics say.
Gaza is controlled by a group called Hamas and is where lots of Palestinian people live. Recently rockets and bombs have been fired into Israel from Gaza.
Israel says it targeted places in Gaza used by Hamas to keep its own people safe from the rocket attacks.
Around 700 people were injured, and Israel says the military action will continue "as long as necessary".
One Israeli was killed by a rocket attack in the town of Netivot, 20 kilometres (12 miles) east of Gaza.
The United Nations has called for all violence in the area to stop.
The UN Secretary-General Ban Ki-moon wants an end to Israel's "excessive use of force leading to the killing and injuring of civilians" and "the ongoing rocket attacks by Palestinian militants".
The rocket attacks on Israel and the Israeli military action come after an agreement not to fight ended.
Israel says it was trying to destroy buildings where Hamas fighters were, places their weapons were being stored and camps used to train them.
At least 30 missiles were fired by F-16 fighter bombers. Israel's Haaretz newspaper reported that about 60 warplanes took part in the first wave of air strikes.
Hamas leaders have said it will launch revenge attacks of its own on Israel.
Things may worsen
Experts in the Middle East are really worried now that the situation is going to get much worse.
The Israeli attacks are the heaviest that many people can remember for some time, and it's thought they could send soldiers in on the ground as well.