List of Field Research tasks and rewards

Please, note that there are always only either: specific amount of stardust, or XP, or just one kind of item, or a particular Pokémon encounter for a single task completed, which means only one of following possible rewards is rewarded for single Field Research task completed not all of them!

Current tasks: May to August 2019
An official e-mail confirmed new field research tasks available in May and June. Shiny Onix, Lileep and Anorith were available since June 4. Shiny Slakoth was available since June 8. Shiny Alolan Exeggutor was available since June 28.

Field Research tasks in July and August were same as those in previous months, with the exception of Spinda #8 replacing #4, and the release of its shiny version.

Catching Pokémon related tasks
Following tasks are related to Catching Pokémon through the Encounter Screen. To complete any of following tasks, Trainer has to successfully capture Pokémon, which is either approached in the wild, from Raid Bonus Challenge or reward encounter for other, already completed Research task.

Throwing Poké Balls related tasks
Following tasks are related with performance of throwing any of Poké Balls when catching Pokémon.

Gym and Raid Battle related tasks
Following tasks are related with battling a Gyms or Raid Battles.

Other tasks
Following tasks are related with Hatching Pokémon Eggs, evolving or power-upping Pokémon, earning Candy with Buddy and other basic game actions.

January to April 2019
Theme for Field Research tasks in January and February 2019 was not told specific in official sources. However, it was noticed that few Field Research tasks added that rewarded Trainers with Pokémon that are essential to catch to progress Let's Go, Meltan Special Research.

Field Research tasks in March and April 2019 were same as those in previous months, with the exception of Spinda #2 replacing #5. Shiny Mankey and Machop were available since March 5. Shiny Lapras was available since April 18.

Catching Pokémon related tasks
Following tasks were related to Catching Pokémon through the Encounter Screen. To complete any of following tasks, Trainer had to successfully capture Pokémon, which was either approached in the wild, from Raid Bonus Challenge or reward encounter for other, already completed Research task.

Throwing Poké Balls related tasks
Following tasks were related with performance of throwing any of Poké Balls when catching Pokémon.

Gym and Raid Battle related tasks
Following tasks were related with battling a Gyms or Raid Battles.

Other tasks
Following tasks were related with Hatching Pokémon Eggs, evolving or power-upping Pokémon, earning Candy with Buddy and other basic game actions.

Event-only tasks
Following Field Research tasks were available during the certain events.

Limited Research tasks
In 2019, there were introduced 3-hour events called Limited Research during which players can obtain exclusive Field Research tasks that completing results with encounter of featured Pokémon.

Community Day tasks
Following is the list of research tasks exclusively available only for the time of specific Community Days.

Hoenn Celebration tasks
Following is the list of research tasks exclusively available only for the time of Hoenn Celebration 2019.

Lunar New Year tasks
Following is the list of research tasks exclusively available only for the time of Lunar New Year 2019.

Valentine's Day tasks
Following is the list of research tasks exclusively available only for the time of Valentine's Day 2019.

Pokémon Day tasks
Following is the list of research tasks exclusively available only for the time of Pokémon Day 2019.

Battle Showdown tasks
Following is the list of research tasks exclusively available only for the time of Battle Showdown 2019.

Equinox tasks
Following is the list of research tasks exclusively available only for the time of Equinox 2019.

Bug Out tasks
Following is the list of research tasks exclusively available only for the time of Bug Out event.

Eggstravaganza tasks
Following is the list of research tasks exclusively available only for the time of Eggstravaganza 2019.

Detective Pikachu Celebration tasks
Following is the list of research tasks exclusively available only for the time of Detective Pikachu Celebration event.

Extraordinary Raid Week
Following is the list of research tasks exclusively available only for the time of Extraordinary Raid Week event.

Adventure Week
Following is the list of research tasks exclusively available only for the time of Adventure Week 2019.

Pokémon GO Fest Dortmund
Following is the list of research tasks exclusively available globally only for the time of Dortmund Pokémon GO Fest 2019.

Trivia

 * If a Trainer encounters a shiny Pokémon as a research reward, said Pokémon will remain shiny until the Trainer captures it. The Trainer is free to run away and re-encounter the Pokémon and it will always be shiny.
 * Upon collecting the reward, you can choose not to enter the encounter immediately, but if you tap the reward again, you will immediately be taken to the encounter without warning. However, if you don't have enough Poké Balls to catch it, you can exit and the reward will still be available later.
 * For Nice/Great throw tasks, a higher tier throw will count.