Index: src/script/interpreter.cpp
===================================================================
--- src/script/interpreter.cpp
+++ src/script/interpreter.cpp
@@ -335,10 +335,9 @@
                 return set_error(serror, SCRIPT_ERR_OP_COUNT);
             }
 
-            if (opcode == OP_CAT || opcode == OP_SUBSTR || opcode == OP_LEFT ||
-                opcode == OP_RIGHT || opcode == OP_INVERT || opcode == OP_2MUL ||
-                opcode == OP_2DIV || opcode == OP_MUL || opcode == OP_LSHIFT ||
-                opcode == OP_RSHIFT) {
+            if (opcode == OP_SUBSTR || opcode == OP_LEFT || opcode == OP_RIGHT ||
+                opcode == OP_INVERT || opcode == OP_2MUL || opcode == OP_2DIV ||
+                opcode == OP_MUL || opcode == OP_LSHIFT || opcode == OP_RSHIFT) {
                 // Disabled opcodes.
                 return set_error(serror, SCRIPT_ERR_DISABLED_OPCODE);
             }
@@ -1234,6 +1233,24 @@
                         }
                     } break;
 
+                    //
+                    // Byte string operations
+                    //
+                    case OP_CAT: {
+                        // (x1 x2 -- out)
+                        if (stack.size() < 2) {
+                            return set_error(
+                                serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
+                        }
+                        valtype& vch1 = stacktop(-2);
+                        valtype& vch2 = stacktop(-1);
+                        if (vch1.size() + vch2.size() > MAX_SCRIPT_ELEMENT_SIZE) {
+                            return set_error(serror, SCRIPT_ERR_PUSH_SIZE);
+                        }
+                        vch1.insert(vch1.end(), vch2.begin(), vch2.end());
+                        stack.pop_back();
+                    } break;
+
                     default:
                         return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
                 }
Index: src/test/data/script_tests.json
===================================================================
--- src/test/data/script_tests.json
+++ src/test/data/script_tests.json
@@ -817,9 +817,6 @@
 ["NOP", "2SWAP 1", "P2SH,STRICTENC", "INVALID_STACK_OPERATION"],
 ["1", "2 3 2SWAP 1", "P2SH,STRICTENC", "INVALID_STACK_OPERATION"],
 
-["'a' 'b'", "CAT", "P2SH,STRICTENC", "DISABLED_OPCODE", "CAT disabled"],
-["'a' 'b' 0", "IF CAT ELSE 1 ENDIF", "P2SH,STRICTENC", "DISABLED_OPCODE", "CAT disabled"],
-
 ["NOP", "SIZE 1", "P2SH,STRICTENC", "INVALID_STACK_OPERATION"],
 
 ["'abc'", "IF INVERT ELSE 1 ENDIF", "P2SH,STRICTENC", "DISABLED_OPCODE", "INVERT disabled"],
Index: src/test/op_code.cpp
===================================================================
--- src/test/op_code.cpp
+++ src/test/op_code.cpp
@@ -304,6 +304,56 @@
         test(script,stack_t{{0xbb,0xf0,0x5d,0x83},{0x4e,0x67,0xab,0x21}},flags,stack_t{{0xbb,0xf0,0x5d,0x83}});
     }
 
+    /// OP_CAT
+
+    void test_cat(uint32_t flags) {
+        CScript script;
+        script << OP_CAT;
+
+        //two inputs required
+        test(script,stack_t(),flags,SCRIPT_ERR_INVALID_STACK_OPERATION);
+        test(script,stack_t{{0x00}},flags,SCRIPT_ERR_INVALID_STACK_OPERATION);
+
+        //stack item with maximum length        
+        item maxlength_item(MAX_SCRIPT_ELEMENT_SIZE, 0x00);
+
+        //Concatenation producing illegal sized output
+        {
+        stack_t input_stack;
+        input_stack.push_back(maxlength_item);
+        item i;
+        i.push_back(0x00);
+        input_stack.push_back(i);
+        test(script,input_stack,flags,SCRIPT_ERR_PUSH_SIZE);
+        }
+    
+        //Concatenation of a max-sized item with empty is legal
+        {
+        stack_t input_stack;
+        input_stack.push_back(maxlength_item);
+        input_stack.push_back(item()); //empty item
+        test(script,input_stack,flags,stack_t{maxlength_item});
+        }
+        {
+        stack_t input_stack;
+        input_stack.push_back(item()); //empty item
+        input_stack.push_back(maxlength_item);
+        test(script,input_stack,flags,stack_t{maxlength_item});
+        }
+
+        //Concatenation of a zero length operand
+        test(script,stack_t{{0x01},{}},flags,stack_t{{0x01}});
+        test(script,stack_t{{},{0x01}},flags,stack_t{{0x01}});
+
+        //Concatenation of two empty operands results in empty item
+        test(script,stack_t{{},{}},flags,stack_t{{}});
+
+        // concatenating two operands generates the correct result
+        test(script,stack_t{{0x00},{0x00}},flags,stack_t{{0x00,0x00}});
+        test(script,stack_t{{0x01},{0x02}},flags,stack_t{{0x01,0x02}});
+        test(script,stack_t{{0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0a},{0x0b,0x0c,0x0d,0x0e,0x0f,0x10,0x11,0x12,0x13,0x14}},flags,stack_t{{0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0a,0x0b,0x0c,0x0d,0x0e,0x0f,0x10,0x11,0x12,0x13,0x14}});
+    }
+
 }
 
 /// Entry points
@@ -345,5 +395,12 @@
     test_mod(STANDARD_LOCKTIME_VERIFY_FLAGS);
 }
 
+BOOST_AUTO_TEST_CASE(op_cat) {
+    test_cat(0);
+    test_cat(STANDARD_SCRIPT_VERIFY_FLAGS);
+    test_cat(STANDARD_NOT_MANDATORY_VERIFY_FLAGS);
+    test_cat(STANDARD_LOCKTIME_VERIFY_FLAGS);
+}
+
 BOOST_AUTO_TEST_SUITE_END()