diff --git a/docs/src/lib/internals/estimator.md b/docs/src/lib/internals/estimator.md
new file mode 100644
index 0000000..d814d4a
--- /dev/null
+++ b/docs/src/lib/internals/estimator.md
@@ -0,0 +1,11 @@
+# Models
+
+## Interface
+
+The interface that has to be implemented for an estimator.
+
+```@autodocs
+Modules = [MetagraphOptimization]
+Pages = ["models/estimator.jl"]
+Order = [:type, :constant, :function]
+```
diff --git a/src/MetagraphOptimization.jl b/src/MetagraphOptimization.jl
index ac0b1ef..ddac405 100644
--- a/src/MetagraphOptimization.jl
+++ b/src/MetagraphOptimization.jl
@@ -121,6 +121,8 @@ include("task/compute.jl")
 include("task/print.jl")
 include("task/properties.jl")
 
+include("estimator/interface.jl")
+
 include("models/interface.jl")
 include("models/print.jl")
 
diff --git a/src/estimator/interface.jl b/src/estimator/interface.jl
new file mode 100644
index 0000000..5d66d4a
--- /dev/null
+++ b/src/estimator/interface.jl
@@ -0,0 +1,25 @@
+
+"""
+    AbstractEstimator
+
+Abstract base type for an estimator. An estimator estimates the cost of a graph or the difference an operation applied to a graph will make to its cost.
+
+Interface functions are
+- [`graph_cost`](@ref)
+- [`operation_effect`](@ref)
+"""
+abstract type AbstractEstimator end
+
+"""
+    graph_cost(estimator::AbstractEstimator, graph::DAG)
+
+Get the total estimated cost of the graph. The cost's data type can be chosen by the implementation, but should have usable comparison operators (<, <=, >, >=, ==) and basic math operators (+, -, *, /).
+"""
+function graph_cost end
+
+"""
+    operation_effect(estimator::AbstractEstimator, graph::DAG, operation::Operation)
+
+Get the estimated effect on the cost of the graph, such that `graph_cost(estimator, graph) + operation_effect(estimator, graph, operation) ~= graph_cost(estimator, graph_with_operation_applied)`. There is no hard requirement for this, but the better the estimate, the better an optimization algorithm will be.
+"""
+function operation_effect end