Throughout your Metal Gear Solid V experience, you'll notice that in order to advance in the game, you'll have to also advance your R&D department, which allows you to research and develop new and more powerful items. Trust me when I say that an early game mission feels completely different when you revisit it later in the game with all new equipment.
As you play, you'll unlock various Key Items which will automatically unlock the ability to research helpful items and upgrades. However, there are a few Key Items which you can miss, as they're either associated with a Side Op, or with a specific condition.
You can check out the entire list of Key Items here, what they unlock and how to obtain them.
Children's Reward – Enables the development of the Water Pistol
Complete Mission 18
First Aid Manual – Enables the development of the Tactical Fulton for D-Dog
Found in Masa Village after you've completed Mission 18
Master Certificate (Marking) – Enables the distinction between real soldiers and decoys when using the binoculars to spot enemies
Mark 500 targets
Master Certificate (Box) – Enables the development of the C. Box WR
Pick up all invoices (fast travel locations) in either Africa or Afghanistan
Grand Master Certificate (Box) – Enables the development of the C. Box SMK
Pick up all invoices (fast travel locations) in both Africa and Afghanistan
Master Certificate (Fulton) – Enables the development of the Grade-3 Fulton
Successfully extract 500 targets
Grand Master Certificate (Fulton) – Enables the development of the F-BALLISTA for D-Walker
Successfully extract 1000 targets
Conch Shell – Enables the development of the Petrol Bomb and the Fulton upgrade to extract children
Complete Side Op 113
Skulls Machete – Enables the development of the TASK-ARM SM for D-Walker
Complete Mission 29
Strangelove's Memento – Enables the development of the Bandana
Complete Side Op 143
Man on Fire (Corpse) – Enables the development of the Furicorn equipment for D-Horse
Complete Side Op 144
Quiet's Examination Report – Enables the development of the Gray XOF equipment for Quiet
Watch the cutscene where Quiet is interrogated, or complete Mission 40
Handkerchief – Enables the development of the Sniper Wolf equipment for Quiet
Complete Mission 40
Emmerich's Research Notes – Enables the development of the Stealth Camo item
Watch the cutscene where Emmerich is exiles
The Codename: Big Boss – Enables the development of the Leather Jacket
Master Certificate (Standard) – Enables the development of the Cyborg Ninja equipment
Complete all Missions (except the high difficulty ones)
Grand Master Certificate (Standard) – Enables the development of the Raiden uniform
Complete all missions with an S Rank (except the high difficulty ones)
Master Certificate (Elite) – Enables the development of the Naked Silver uniform and Silver Bionic Arm
Complete all high difficulty missions
Grand Master Certificate (Elite) – Enables the development of the Naked Gold uniform
Complete all high difficulty missions with an S Rank
Star of Bethlehem – Enables the development of the Infinity Bandana
