The annual report analyzes the efforts of 188 countries to comply with minimum standards needed to eliminate trafficking of men, women and children for sexual exploitation or forced labor. It rates each nation's efforts according to a Tier system -- Tier 1 being most compliant, through to Tier 3 being the worst offenders.
So which countries are in the firing line to fall in status, and which ones have improved?
Tier 2 is the most volatile category because it includes a U.S. State Department Watch List for countries that are deemed to not be doing enough to counter human traffickers.
Last year, 89 countries were listed as Tier 2. Another 44 countries were on the Tier 2 Watch List -- a warning that unless more was done, they could be downgraded to Tier 3 -- a category for the least compliant nations.
Penalties for countries demoted to Tier 3 are at the discretion of the U.S. President, but could include restrictions on non-humanitarian assistance and funding.
Who fell to Tier 3 last year?
Thailand, Malaysia, Venezuela and The Gambia dropped from the Tier 2 Watch List to join 20 other nations on Tier 3 in 2014.
Thailand was downgraded for its failure to do enough to stop women and children being trafficked through and into the country for exploitation in the sex trade. Men were being sold into slavery on fishing boats where they spent months at sea in harsh conditions without pay, the report said.
In recent months, the country's military government has appeared to try to address the problem, and on Friday, Thai authorities announced they'd charged 72 people, including government and military officials, with human trafficking crimes.
It follows a number of raids on migrant camps along the Thailand-Malaysia border that uncovered mass graves believed to hide the remains of dead Bangaldeshi migrants, and Muslim Rohingya from Myanmar. In recent years, around 100,000 Rohingya are believed to have fled ethnic cleansing in Rakhine State, Myanmar, which is also known as Burma.
However, the raids -- in Thailand
-- came months after the March 2015 cut-off date for this year's report, so are unlikely to have influenced their rating this year. Instead, they'll be considered for the 2016 report, spokeswoman for the U.S. Embassy in Thailand Melissa Sweeney told Thai media.
Both Thailand and Malaysia were given automatic downgrades to Tier 3 after two years on the Tier 2 Watch List. Under new rules introduced in 2013, countries can spent a maximum two years list on the Watch List before they have to move up to Tier 2 or down to Tier 3. No further waivers are allowed.
Myanmar: Listed a source country for trafficking -- has spent three consecutive years on the Tier 2 Watch List after being granted a waiver from Tier 3 based on its plans to address the issue. The new rules -- and pressure from activists -- put it in line for a potential downgrade. Bangladesh was listed as Tier 2 last year.
China: Listed as a source, transit and destination country for human trafficking, China was upgraded last year from Tier 3 to the Tier 2 Watch List. The report said that while the Chinese government "does not fully comply with the minimum standards for the elimination of trafficking," it was making "significant efforts to do so."
The report cited China's move to close a number of "Reform through labor" camps, where prisoners are forced to work, sometimes for years, without pay, as one reason for its promotion. It also said a relaxation of the one-child policy would likely reduce future demand for prostitution and foreign brides for Chinese men.
Qatar: Last year, Qatar dropped to the Tier 2 Watch List because the government "did not demonstrate evidence of overall increasing efforts to address human trafficking since the previous reporting period." Qatar has come under fire for its treatment of migrant workers in the lead up to the 2022 World Cup, amid a corruption scandal at the heart of the world football governing body, FIFA.
What nations are actively addressing human trafficking?
Of the 200 countries surveyed by the TIP report, only 31 countries including Australia, the U.S., UK, Sweden, Canada, South Korea, Armenia, Taiwan, Nicaragua and Chile were listed as Tier 1 countries.
It's not a gold star rating saying there is no trafficking in these countries -- even Australia
has earned worldwide criticism for its policy of turning back boats carrying refugees
from the Middle East and Sri Lanka, to Indonesia. Rather, nations on this list are making efforts to address the problem and comply with U.S. legislation relating to its Trafficking Victims Protection Reauthorization Act.
Look for numbers on prosecutions
Another indicator to look for is statistics showing prosecutions globally. These indicate if countries are making progress in reducing human trafficking in their jurisdictions.
Statistics for human trafficking vary wildly mainly due to the global size of the problem and a lack of reliable reporting. According to Human Rights First, there are roughly 21 million victims of slavery worldwide in an industry netting $150 billion profit for traffickers.
However, the Global Slavery Index
claims there are currently 35.8 million people subjected to modern slavery. It says around 14 million of those -- nearly a third of victims -- are enslaved in India
via forced labor or human trafficking.
Last year, India was listed as a Tier 2 country on the TIP report, which said the country was "making significant efforts" to combat trafficking though prosecution of offenders and protection of victims was "uneven" across states.
Despite the millions of people enduring modern slavery, the figures for prosecutions are very low. According to the last TIP Report, 44,758 victims were identified in 2013, resulting in only 9,460 prosecutions and 5,776 convictions.