diff --git a/src/interpreter.c b/src/interpreter.c
index 00e5a0a..289544c 100644
--- a/src/interpreter.c
+++ b/src/interpreter.c
@@ -3,6 +3,7 @@
 #include "types.h"
 #include "include/uthash.h"
 #include <stdlib.h>
+#include <string.h>
 
 int currentInstruction = 0;
 
@@ -21,6 +22,14 @@ void runtimeError(GroundRuntimeError error, char* what, GroundInstruction* where
             printf("TooManyArgs");
             break;
         }
+        case UNKNOWN_LABEL: {
+            printf("UnknownLabel");
+            break;
+        }
+        case UNKNOWN_VARIABLE: {
+            printf("UnknownVariable");
+            break;
+        }
         default:
         case FIXME: {
             printf("FIXME (please report issue to https://chsp.au/ground/cground)");
@@ -98,6 +107,18 @@ void interpretGroundProgram(GroundProgram* in) {
 }
 
 void interpretGroundInstruction(GroundInstruction* in, GroundScope* scope) {
+    // Insert variables prefixed with $
+    for (int i = 0; i < in->args.length; i++) {
+        if (in->args.args[i].type == VALREF) {
+            GroundVariable* variable = findVariable(*scope->variables, in->args.args[0].value.refName);
+            if (variable) {
+                in->args.args[i].value.value = variable->value;
+                in->args.args[i].type = VALUE;
+            } else {
+                runtimeError(UNKNOWN_VARIABLE, NULL, in, currentInstruction);
+            }
+        }
+    }
     switch (in->type) {
         // We can safely ignore any CREATELABEL instructions, these have been preprocessed
         case CREATELABEL: {
@@ -156,7 +177,7 @@ void interpretGroundInstruction(GroundInstruction* in, GroundScope* scope) {
         }
         case PRINT: {
             if (in->args.length < 1) {
-                runtimeError(TOO_FEW_ARGS, NULL, in, currentInstruction);
+                runtimeError(TOO_FEW_ARGS, "Expecting 1 or more args", in, currentInstruction);
             }
             for (int i = 0; i < in->args.length; i++) {
                 printGroundArg(&in->args.args[i]);
@@ -166,7 +187,7 @@ void interpretGroundInstruction(GroundInstruction* in, GroundScope* scope) {
         }
         case PRINTLN: {
             if (in->args.length < 1) {
-                runtimeError(TOO_FEW_ARGS, NULL, in, currentInstruction);
+                runtimeError(TOO_FEW_ARGS, "Expecting 1 or more args", in, currentInstruction);
             }
             for (int i = 0; i < in->args.length; i++) {
                 printGroundArg(&in->args.args[i]);
@@ -175,6 +196,85 @@ void interpretGroundInstruction(GroundInstruction* in, GroundScope* scope) {
             printf("\n");
             break;
         }
+
+        case SET: {
+            if (in->args.length < 2) {
+                runtimeError(TOO_FEW_ARGS, "Expecting 2 args", in, currentInstruction);
+            }
+            if (in->args.length > 2) {
+                runtimeError(TOO_MANY_ARGS, "Expecting 2 args", in, currentInstruction);
+            }
+            if (in->args.args[0].type != DIRREF) {
+                runtimeError(ARG_TYPE_MISMATCH, "Expecting a DirectRef for arg 1", in, currentInstruction);
+            }
+            if (in->args.args[1].type != VALUE) {
+                runtimeError(ARG_TYPE_MISMATCH, "Expecting a Value for arg 2", in, currentInstruction);
+            }
+
+            addVariable(scope->variables, in->args.args[0].value.refName, in->args.args[1].value.value);
+            break;
+        }
+
+        case ADD: {
+            if (in->args.length < 3) {
+                runtimeError(TOO_FEW_ARGS, "Expecting 3 args", in, currentInstruction);
+            }
+            if (in->args.length > 3) {
+                runtimeError(TOO_MANY_ARGS, "Expecting 3 args", in, currentInstruction);
+            }
+            if (in->args.args[0].type != VALUE) {
+                runtimeError(ARG_TYPE_MISMATCH, "Expecting a Value for arg 1", in, currentInstruction);
+            }
+            if (in->args.args[1].type != VALUE) {
+                runtimeError(ARG_TYPE_MISMATCH, "Expecting a Value for arg 2", in, currentInstruction);
+            }
+            if (in->args.args[2].type != DIRREF) {
+                runtimeError(ARG_TYPE_MISMATCH, "Expecting a DirectRef for arg 3", in, currentInstruction);
+            }
+
+            GroundValue* left = &in->args.args[0].value.value;
+            GroundValue* right = &in->args.args[1].value.value;
+
+            if (left->type == STRING) {
+                if (right->type != STRING) {
+                    runtimeError(ARG_TYPE_MISMATCH, "Expecting a String for arg 2", in, currentInstruction);
+                }
+                char* newString = malloc(sizeof(char));
+                strcpy(newString, left->data.stringVal);
+                strcat(newString, right->data.stringVal);
+
+                addVariable(scope->variables, in->args.args[2].value.refName, createStringGroundValue(newString));
+            }
+            else if (left->type == INT || left->type == DOUBLE) {
+                if (right->type != INT && right->type != DOUBLE) {
+                    runtimeError(ARG_TYPE_MISMATCH, "Expecting an Int or Double for arg 2", in, currentInstruction);
+                }
+
+                if (left->type == DOUBLE || right->type == DOUBLE) {
+                    double result = 0;
+                    
+                    if (left->type == INT) {
+                        result += left->data.intVal;
+                    } else if (left->type == DOUBLE) {
+                        result += left->data.doubleVal;
+                    }
+    
+                    if (right->type == INT) {
+                        result += right->data.intVal;
+                    } else if (left->type == DOUBLE) {
+                        result += right->data.doubleVal;
+                    }
+
+                    addVariable(scope->variables, in->args.args[2].value.refName, createDoubleGroundValue(result));
+                } else {
+                    int64_t result = left->data.intVal + right->data.intVal;
+                    addVariable(scope->variables, in->args.args[2].value.refName, createIntGroundValue(result));
+                }
+            } else {
+                runtimeError(ARG_TYPE_MISMATCH, "Expecting an Int, Double, or String for arg 1", in, currentInstruction);
+            }
+            break;
+        }
         default: {
             runtimeError(FIXME, "Currently unimplemented instruction", in, currentInstruction);
         }
diff --git a/src/interpreter.h b/src/interpreter.h
index 36a8026..06c677f 100644
--- a/src/interpreter.h
+++ b/src/interpreter.h
@@ -7,7 +7,7 @@
 #include "include/uthash.h"
 
 typedef enum GroundRuntimeError {
-    ARG_TYPE_MISMATCH, TOO_FEW_ARGS, TOO_MANY_ARGS, UNKNOWN_LABEL, FIXME 
+    ARG_TYPE_MISMATCH, TOO_FEW_ARGS, TOO_MANY_ARGS, UNKNOWN_LABEL, UNKNOWN_VARIABLE, FIXME 
 } GroundRuntimeError;
 
 typedef struct GroundLabel {
diff --git a/tests/simple.grnd b/tests/simple.grnd
index 002ef14..71d9ef2 100644
--- a/tests/simple.grnd
+++ b/tests/simple.grnd
@@ -1,3 +1,19 @@
-@myLabel
-println "dingus"
-jump %myLabel
+# Check setting variables
+set &myVar "dingus"
+println $myVar
+
+# Check string concat
+add "dingle" "fart" &myOtherVar
+println $myOtherVar
+
+# Check double math
+add 3.14 2.7 &myThirdVar
+println $myThirdVar
+
+# Check int math
+add 464398727 374298 &yetAnotherVar
+println $yetAnotherVar
+
+# Check mixed math
+add 432 4732.12 &finalVar
+println $finalVar