The Theory of Derivations in Almost Distributive Lattices is A function d : 𝐋 ⟶ 𝐋 is called a derivation on L, if 𝑑(𝑥 ∧ 𝑦) = (𝑑𝑥 ∧ 𝑦) ∨ (𝑥 ∧ 𝑑𝑦), 𝑓𝑜𝑟 𝑎𝑙𝑙 𝑥, 𝑦 ∈ 𝐿. A principal derivation is an ADL and 𝑎 ∈ 𝐿. Define a function 𝑑𝑎 on 𝐿 by 𝑑𝑎𝑥 = 𝑎 ∧ 𝑥 , for all 𝑥 ∈ 𝐿. Then, 𝑑𝑎 is a derivation on 𝐿.and . a derivation d on L is an isotone derivation, if 𝑑𝑎 ≤ 𝑑𝑏 for all 𝑎, 𝑏 ∈ 𝐿 with 𝑎 ≤ 𝑏. Keywords: Almost Distributive Lattices the fixed set of a derivation, isotone derivation