Less than a week before Christmas, thousands of Amazon warehouse workers across the country are on strike after the company refused to enter into contract negotiations with the International Brotherhood of Teamsters.

Although the first Amazon warehouse union in New York City was formed well over two years ago and workers at several other warehouses across the country have begun organizing, the company refuses to recognize the union and negotiate a collective bargaining agreement with it .

The Teamsters gave Amazon a Dec. 15 deadline to come to the negotiating table, but the company refused to acknowledge that.

Here’s what you need to know about the Amazon warehouse strike.

Which Amazon workers are on strike?

Amazon employs more than 1.5 million people worldwide. The Amazon Labor Union (ALU) has recruited approximately 10,000 workers across 10 plants across the US over the past two years. In June, the ALU voted to join the Teamsters, which has 1.3 million union members nationwide.

The Teamsters said seven of the 10 facilities with unions are on strike. The warehouses include one in the borough of Queens, New York, one in Atlanta, three in Southern California, one in San Francisco and a seventh in a Chicago suburb. The Teamsters said Amazon Teamsters at other facilities are “ready to join them.”

According to the union, this is the largest strike against Amazon in the company’s history.

How will the strike affect deliveries?

It is unclear how the strike will affect operations. Amazon still has hundreds of thousands of employees working in non-union delivery centers and facilities.

The company said it does not expect the strike to affect its operations until Christmas.

But Teamsters President Sean O’Brien said in a statement Thursday morning that Amazon’s insatiable greed may be to blame if Amazon customers experience package delays.

“We have given Amazon a clear deadline to come to the table and do the right thing for our members. They ignored it,” he said. “These greedy executives have had every chance to show decency and respect for the people who make their obscene profits possible. Instead, they have pushed workers to their limits and are now paying the price. The strike is on them.”

The Teamsters said their local unions would also picket lines at hundreds of other non-union Amazon fulfillment centers across the country.

How long will the strike last?

The Teamsters have not indicated how long the strike will last, meaning it could last past Christmas and into the New Year.

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Why are Amazon workers striking?

Although thousands of its employees have voted to unionize, the company has strongly opposed efforts to increase union membership and refused to recognize some employees as Amazon workers.

The union wants Amazon to negotiate a contract with its members, who demand better wages, job security and benefits.

Amazon warehouse workers and delivery drivers have long reported conditions that can be stressful for workers. A report released this week by a U.S. Senate committee led by Democratic Sen. Bernie Sanders accused Amazon of subjecting workers to high quotas that jeopardized their safety. The committee said Amazon warehouses experienced 30% more injuries compared to the industry average. Amazon said the report was “fundamentally flawed.”

Why isn’t Amazon negotiating with the union?

Some Amazon Teamster members are delivery drivers who work for third-party companies called “delivery service partners” and drive vans with Amazon logos, but are not technically employed by the company.

Earlier this year, two regional directors of the National Labor Relations Board (NLRB) filed initial complaints that Amazon was technically a co-employer of the drivers, even if they were hired by third parties. A hearing on an NLRB complaint in Southern California is scheduled for March.

In September, Amazon filed its own complaint against the NLRB’s structure, arguing it was unconstitutional because board members cannot be removed by the president. Although the company, along with Elon Musk’s SpaceX, which filed a similar lawsuit, argued before a panel of judges at the U.S. Court of Appeals for the Fifth Circuit in November, the court has not yet issued a ruling.

But that means Amazon’s opposition to union recognition is still entrenched in court, with the company claiming that many of its unionized members are not its employees and that the union is harassing and intimidating employees.

“For more than a year, the Teamsters have intentionally misled the public by claiming they represent ‘thousands of Amazon employees and drivers.’ They are not doing that, and this is another attempt to spread a false narrative,” an Amazon spokesperson said in a statement.