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! 

April 2018 tasks
Theme for Field Research tasks in April 2018 was Fire-type Pokémon.

May 2018 tasks
Theme for Field Research tasks in May 2018 was Electric- and Flying-type Pokémon.

June 2018 tasks
Theme for Field Research tasks in June 2018 was Water-type Pokémon.

July 2018 tasks
Theme for Field Research tasks in July 2018 was Kanto-region Pokémon.

August 2018 tasks
Theme for Field Research tasks in August 2018 was Electric-type Pokémon.

September 2018 tasks
Theme for Field Research tasks in September 2018 is Fire-type Pokémon.

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

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

Adventure Week 2018 tasks
Following is the list of research tasks exclusively available only for the time of Adventure Week in 2018. There was a possibility to encounter shiny variants of Omanyte, Kabuto and Aerodactyl as the reward for these tasks.

Water Festival 2018 tasks
Following is the list of research tasks exclusively available only for the time of Water Festival in 2018. There was a possibility to encounter shiny variants of Shellder, Magikarp and Wailmer as the reward for these tasks. The last two tasks were added in the latter half of the event.

GO Fest 2018 tasks
Following is the list of research tasks exclusively available only for the time of GO Fest in 2018. It was available only for attendants of the Fest in Chicago, Illinois, on July and, 2018. As seen all 4 of the tasks rewarded trainers with a single Rare Candy.

Johto Festival 2018 tasks
Following is the list of research tasks exclusively available only for the time of Johto Festival in 2018. These tasks gives Trainers opportunity to complete their Johto Pokédexes and complete A Ripple in Time, the Special Research tasks series. There was a possibility to encounter shiny variants of Pineco as the reward for these tasks. Additionally, these tasks for the first time in game's history rewarded Trainers for completing them with Evolution Items.

Pokémon World Championships 2018
During the time of Pokémon World Championships in Nashville, that took place in the later part of August, 2018, there was a single Field Research task available from PokéStops in 1 mile from the Championships.

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.