Representatives of the European Union and the United Kingdom gathered in Brussels Monday to resume Brexit negotiations. The talks, slated to run through Thursday, are expected to cover everything the U.K.’s financial obligations to the standing of Northern Ireland. “For us it’s incredibly important we now make good progress,” David Davis, the U.K. Brexit secretary, told reporters Monday at a joint press conference with EU Chief Brexit Negotiator Michel Barnier, adding that both sides would work to “identify the differences so that we can deal with them and identify the similarities so we can reinforce them.”

Of all the differences looming over the talks, none has emerged as more contentious in recent weeks than the fate of the 3.5 million EU citizens living in the U.K. and 1.2 million British nationals living in Europe, and the rights each can expect to retain once the divorce is finalized. These rights and protections, such as freedom of movement and residence, have been billed as “the first priority” for EU leaders. “I would like to state very clearly that we need real guarantees for our people who live, work, and study in the U.K., and the same goes for the Brits,” Donald Tusk, the president of the European Council, told reporters following an EU Council meeting in April.

The British government expressed similar sentiment, unveiling last month what Prime Minister Theresa May called a “fair and serious” proposal for the country’s EU citizens post-Brexit. Under the proposal, these individuals would be guaranteed new rights under U.K. law, most notable of which is the ability to earn “settled status” after completing five years of U.K. residence, giving those who qualify access to public funds and services, as well as the option to apply for U.K. citizenship. This plan would only apply to EU nationals living in the U.K. prior to a yet to be determined cut-off date between March 29, the day the U.K. formally triggered Article 50 of the Lisbon Treaty to begin the process of withdrawal from the bloc, and the day Britain formally leaves.

The U.K. proposal is contingent on reciprocal rights for Britons living within the EU, but EU leaders say the U.K.’s guarantees fall short of those proposed by the EU for British nationals. Unlike the EU proposal that allows Britons in Europe to bring their family members in perpetuity, the British proposal would make it so that EU citizens could no longer bring their non-EU family members to the U.K. unless they meet a minimum income threshold (though British citizens already adhere to this requirement, EU nationals previously enjoyed fewer restrictions). Additionally, while the EU proposal guarantees the existing rules that protect Britons’ access to benefits, healthcare, and education, the U.K. proposal provides no such guarantees.

These discrepancies prompted Barnier to call on the U.K. to offer “more ambition, clarity, and guarantees.” Guy Verhofstadt, the European Parliament’s Brexit coordinator, took the criticism one step further, dubbing the U.K. proposal a “damp squib” that reduces Europeans to “the status of ‘third-country nationals’ in the U.K., with fewer rights than British citizens are offered throughout the EU.”

Kenan Hadzimusic, a senior manager at the European Citizens Action Service (ECAS), a Brussels-based NGO focusing on citizens’ rights, told me the concerns both U.K. and EU citizens have stem from a lack of clarity over how their rights may be affected once the Brexit deal is done. Indeed, a June study by the ECAS found that freedom of movement stood as the primary concern for both groups of people. When analyzed individually, however, Britons were found to be most concerned with losing their European citizenship, whereas EU citizens were most concerned with uncertainty about the future, xenophobia, and discrimination.

“What will be much fairer and easier would be to in fact look at those rights that [EU citizens] have acquired and keep them, and have a reciprocal solution for the U.K. citizens living in other countries of the EU,” Hadzimusic said. “That is what the European Union Commission negotiating team is also proposing.” These issues, he said, go beyond freedom of movement within the continent. “It will have an affect on people’s lives and their professional career, possibilities to study or not in other countries, to live with their spouses.”

He added regarding the number of people that would be affected by the policy: “That’s the population of Croatia, for example, which is one member of the EU. It’s as if you’re looking at the population of one country.”

One solution the EU proposed to address some of the concerns over EU citizens’ rights is to allow the European Court of Justice (ECJ), the EU’s high court, to have jurisdiction over cases that originate in U.K. courts before it formally leaves the bloc. Steven Peers, a professor of law at the University of Essex, told me this aims to ensure EU citizens’ rights don’t change even if U.K. law does. “They don’t necessarily trust the U.K. and they want to have the EU court there as a kind of independent body to ensure that citizens’ rights are still protected,” he said.

Maintaining ECJ jurisdiction has long been considered a non-starter for the British government. Indeed, May reaffirmed in a January speech outlining the U.K.’s Brexit strategy that “we will not have truly left the European Union if we are not in control of our laws.” But that doesn’t mean the U.K. isn’t open to compromise. May’s spokesman announced last week that “the transition rules could involve the ECJ for a limited time.”

Benoît Dillet, a teaching fellow at the University of Bath, told me this would be no small concession. “This rejection of the ECJ as [a] mediator comes from the U.K. government’s rhetorical and strategic call for ‘taking back control,’” he said. “If they manage to reach a compromise, then it would show the strength of EU institutions, since Theresa May has been unilateral on this issue for years.”

Though citizens’ rights will be one of the primary challenges facing negotiators, it won’t be the only one. The demand by EU leaders that Britain pay upwards of 60 billion euros to leave the EU has threatened to derail talks before they’ve even begun. “The divorce bill is more likely to lead to the collapse of talks than anything else,” Peers said. “I assume it will end up being a bigger gap than we have in the area of citizens’ rights.” Indeed, U.K. Foreign Secretary Boris Johnson told Parliament Tuesday that the EU could “go whistle” if they expected the U.K. to pay the exit toll—a remark that was met with little enthusiasm by EU leaders.

“I am not hearing any whistling,” Barnier responded Wednesday. “Just the clock ticking.”