If Egyptian unrest turns into an Egyptian revolution, the implications for the Arab world - and for Western policy in the Middle East - will be immense.
Egypt matters, in a way that tiny Tunisia - key catalyst that it has been in the current wave of protest - does not.
Egypt, the most populous Arab state, can help determine the thrust of Arab policies - whether towards Israel or Iran or in the perennial quest for Arab consensus on issues that matter.
Above all, the Egyptian state has traditionally had a strength and solidity that made its collapse seem unthinkable.
Even now, with so much that is uncertain, that state and its basic structures may survive - with or without Hosni Mubarak, the country's president for the last three decades.
Islamist wild card
If there is a power vacuum, who is likely to fill it?
Will the powerful military intervene to restore stability?
If they did, would the protesters accept such a scenario - or would they, like their Tunisian counterparts, keep up the pressure for radical change?
And - the wild card that troubles Western policy-makers most - could the Muslim Brotherhood, the country's Islamist opposition movement, somehow exploit the protests to come to power?
Right now, that scenario seems far-fetched. The Brotherhood is trying to jump on the bandwagon of a youthful and largely leaderless protest movement.
They are not in front. They are trying to catch up.
But the situation is volatile. New leaders - nationalist or Islamist, civilian or military - could emerge if the country is engulfed in chaos.
If the Mubarak regime were to collapse - which is still a big "if" - the fall-out would affect virtually every key player in the region and every key issue.
•For Arab autocrats, it would signify the writing on the wall in a far more dramatic way than the fall of the Ben Ali regime in Tunisia.
•For Arab protesters, it would be a great boost, fuelling the idea that the region has entered a new era of "people power".
•It would deal a blow to an already enfeebled Middle East peace process. Egypt was the first Arab state to sign a peace treaty with Israel, back in the 1970s. A change of regime would alarm Israeli leaders and deepen the siege mentality among many Israelis.
•It would affect business confidence, regionally and even globally, especially if oil prices shot up.
•Finally, it would pose painful dilemmas for Western policy-makers who have long favoured gradual political reform in the region, fearful that the alternative could be the breakdown of stability and the rise of extremism.
Right now, Arab rulers and Arab citizens are glued to their TV screens, computers and mobile phones for news of how the drama is unfolding.
It will be some time before the smoke and tear gas settle, and the new face of this troubled region begins to come into focus.
Roger Hardy is a Middle East analyst at the Woodrow Wilson Center, Washington, DC