diff --git a/cef-test-core/src/harness/feedback.rs b/cef-test-core/src/harness/feedback.rs
index 71e14d1..9957894 100644
--- a/cef-test-core/src/harness/feedback.rs
+++ b/cef-test-core/src/harness/feedback.rs
@@ -8,6 +8,8 @@ pub enum Feedback {
     AssertFailure,
     /// Last instruction raised an error
     Error,
+    /// Last instruction was not supported by adaptor
+    Unsupported,
 }
 
 impl Feedback {
diff --git a/cef-test-core/src/harness/harness.rs b/cef-test-core/src/harness/harness.rs
index 99a68da..c2460f2 100644
--- a/cef-test-core/src/harness/harness.rs
+++ b/cef-test-core/src/harness/harness.rs
@@ -1,5 +1,5 @@
 use super::{TestRunner, TestAdaptor, TestMetadata};
-use super::{Instruction, TestAssert, UIOp, Feedback};
+use super::{Instruction, TestAssert, TestOp, Feedback};
 
 /// Harness which runs one or more tests
 pub struct TestHarness<R: TestRunner, A: TestAdaptor> {
@@ -21,7 +21,7 @@ impl<R: TestRunner, A: TestAdaptor> TestHarness<R, A> {
         Feedback::Success
     }
 
-    fn translate_ui_op(&self, _op: UIOp) -> Feedback {
+    fn translate_ui_op(&self, _op: TestOp) -> Feedback {
         // TODO
         Feedback::Success
     }
@@ -29,7 +29,7 @@ impl<R: TestRunner, A: TestAdaptor> TestHarness<R, A> {
     fn translate_instruction(&self, instruction: Instruction) -> Feedback {
         match instruction {
             Instruction::Assertion(a) => self.translate_assertion(a),
-            Instruction::Interaction(i) => self.translate_ui_op(i),
+            Instruction::Operation(i) => self.translate_ui_op(i),
         }
     }
 
diff --git a/cef-test-core/src/harness/instructions.rs b/cef-test-core/src/harness/instructions.rs
index 917f5be..039975a 100644
--- a/cef-test-core/src/harness/instructions.rs
+++ b/cef-test-core/src/harness/instructions.rs
@@ -2,12 +2,91 @@
 pub enum Instruction {
     /// Test assertion
     Assertion(TestAssert),
-    /// UI manipulation
-    Interaction(UIOp),
+    /// Test harness operation
+    Operation(TestOp),
 }
 
 /// Assertion
-pub enum TestAssert {}
+pub struct TestAssert {
+    /// Tab context
+    pub context: TabSelector,
+    /// Test assertion
+    pub assertion: GeneralAssertType,
+}
+
+/// Test assertion information
+pub enum GeneralAssertType {
+    /// Element-related assertion
+    Element(ElementAssert),
+}
+
+/// Element assertion
+pub struct ElementAssert {
+    /// Element to target
+    pub element: ElementSelector,
+    /// Test assertion
+    pub assert: ElementAssertionType,
+}
+
+/// Assertion operations
+pub enum ElementAssertionType {
+    /// Assert element exists
+    Exists,
+    /// Assert element contains text
+    TextEquals(String)
+}
 
 /// User interface interaction
-pub enum UIOp {}
+pub struct TestOp {
+    /// Tab context
+    pub context: TabSelector,
+    /// Test operation
+    pub op: GeneralOpType,
+}
+
+/// Element selection mode
+pub enum ElementSelector {
+    /// Use CSS selector syntax
+    CSS(String),
+}
+
+/// Tab selection mode
+pub enum TabSelector {
+    /// Select by tab title
+    Title(String),
+    /// Select by tab's current URL
+    Url(String),
+    /// Select by tab identifier
+    Id(String),
+}
+
+/// Test operation information
+pub enum GeneralOpType {
+    /// Operate on an element
+    Element(ElementOp),
+    /// Basic context operation
+    Basic(BasicOpType),
+}
+
+/// Basic operation type
+pub enum BasicOpType {
+    /// Pause executing thread for time, in milliseconds
+    Sleep(u64),
+}
+
+/// Element manipulation operation
+pub struct ElementOp {
+    /// Element to target
+    pub element: ElementSelector,
+    /// Operation to perform
+    pub op: ElementOpType,
+}
+
+/// Element operation type
+pub enum ElementOpType {
+    /// Click on element
+    Click,
+    /// Wait for element to be created
+    WaitFor,
+}
+
diff --git a/cef-test-core/src/harness/mod.rs b/cef-test-core/src/harness/mod.rs
index 7598278..2867784 100644
--- a/cef-test-core/src/harness/mod.rs
+++ b/cef-test-core/src/harness/mod.rs
@@ -13,6 +13,6 @@ pub use adaptor::TestAdaptor;
 pub use feedback::Feedback;
 pub use harness::TestHarness;
 pub use headless_adaptor::HeadlessAdaptor;
-pub use instructions::{Instruction, TestAssert, UIOp};
+pub use instructions::{Instruction, TestAssert, GeneralAssertType, ElementAssert, ElementAssertionType, TestOp, ElementSelector, TabSelector, GeneralOpType, BasicOpType, ElementOp, ElementOpType};
 pub use json_runner::JsonRunner;
 pub use runner::{TestRunner, TestMetadata};