Restricted access to water and other serious problems with infrastructure has put pressure on residents of Gaza.
But on Monday the Gaza Strip was decked out in green - the colour most associated with Hamas.
Celebrations last year drew tens of thousands onto the streets.
Just weeks later, Israel launched Operation Cast Lead, an air and ground attack it said was aimed at stopping rockets being fired from Gaza.
In November this year Hamas said it would cease firing rockets unless Israel attacked, but other groups have continued to launch sporadic attacks on Israel.
"In the 22 years since its founding, Hamas has been able to realise a large part of its goals and to overcome every obstacle it has faced, from prison, exile, assassinations and elections," senior Hamas leader Mahmoud Zahar said.
Sheikh Yassin was killed in an Israeli missile strike in 2004.
Nearly a year after the conflict with Israel, Gaza is still struggling to recover.
Israeli military action destroyed thousands of homes, hundreds of factories and 80 official buildings in the strip.
Palestinians and human rights groups say more than 1,400 people were killed in the violence between 27 December 2008 and 16 January 2009, more than half of them civilians.
Israel puts the number of deaths at 1,166 - fewer than 300 of them civilians. Three Israeli civilians and 10 Israeli soldiers were also killed.
Hamas, an Arabic acronym for the Islamic Resistance Movement, was created in 1987 after the beginning of the first intifada, or Palestinian uprising, against Israel's occupation of the West Bank and Gaza.
Hamas is designated a terrorist organisation by Israel, the US and the EU, but it is seen by its supporters as a legitimate resistance movement.
Sporadic discussions between Hamas and Fatah to resolve the two factions' dispute are at an impasse.
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