diff --git a/alternator-test/test_expected.py b/alternator-test/test_expected.py
index acdc711301..29d40fea41 100644
--- a/alternator-test/test_expected.py
+++ b/alternator-test/test_expected.py
@@ -524,7 +524,6 @@ def test_update_expected_1_null(test_table_s):
         )
 
 # Tests for Expected with ComparisonOperator = "CONTAINS":
-@pytest.mark.xfail(reason="ComparisonOperator=CONTAINS in Expected not yet implemented")
 def test_update_expected_1_contains(test_table_s):
     # true cases. CONTAINS can be used for two unrelated things: check substrings
     # (in string or binary) and membership (in set or list).
@@ -607,7 +606,6 @@ def test_update_expected_1_contains(test_table_s):
         )
 
 # Tests for Expected with ComparisonOperator = "NOT_CONTAINS":
-@pytest.mark.xfail(reason="ComparisonOperator=NOT_CONTAINS in Expected not yet implemented")
 def test_update_expected_1_not_contains(test_table_s):
     # true cases. NOT_CONTAINS can be used for two unrelated things: check substrings
     # (in string or binary) and membership (in set or list).
diff --git a/alternator/conditions.cc b/alternator/conditions.cc
index c8a5e530cd..6f4fcc1995 100644
--- a/alternator/conditions.cc
+++ b/alternator/conditions.cc
@@ -29,6 +29,7 @@
 #include "rjson.hh"
 #include "serialization.hh"
 #include "base64.hh"
+#include <stdexcept>
 
 namespace alternator {
 
@@ -47,7 +48,9 @@ comparison_operator_type get_comparison_operator(const rjson::value& comparison_
             {"NOT_NULL", comparison_operator_type::NOT_NULL},
             {"BETWEEN", comparison_operator_type::BETWEEN},
             {"BEGINS_WITH", comparison_operator_type::BEGINS_WITH},
-    }; //TODO: CONTAINS
+            {"CONTAINS", comparison_operator_type::CONTAINS},
+            {"NOT_CONTAINS", comparison_operator_type::NOT_CONTAINS},
+    };
     if (!comparison_operator.IsString()) {
         throw api_error("ValidationException", format("Invalid comparison operator definition {}", rjson::print(comparison_operator)));
     }
@@ -179,6 +182,59 @@ static bool check_BEGINS_WITH(const rjson::value* v1, const rjson::value& v2) {
     return val1.substr(0, val2.size()) == val2;
 }
 
+static std::string_view to_string_view(const rjson::value& v) {
+    return std::string_view(v.GetString(), v.GetStringLength());
+}
+
+static bool is_set_of(const rjson::value& type1, const rjson::value& type2) {
+    return (type2 == "S" && type1 == "SS") || (type2 == "N" && type1 == "NS") || (type2 == "B" && type1 == "BS");
+}
+
+// Check if two JSON-encoded values match with the CONTAINS relation
+static bool check_CONTAINS(const rjson::value* v1, const rjson::value& v2) {
+    if (!v1) {
+        return false;
+    }
+    const auto& kv1 = *v1->MemberBegin();
+    const auto& kv2 = *v2.MemberBegin();
+    if (kv2.name != "S" && kv2.name != "N" &&  kv2.name != "B") {
+        throw api_error("ValidationException",
+                        format("CONTAINS operator requires a single AttributeValue of type String, Number, or Binary, "
+                               "got {} instead", kv2.name));
+    }
+    if (kv1.name == "S" && kv2.name == "S") {
+        return to_string_view(kv1.value).find(to_string_view(kv2.value)) != std::string_view::npos;
+    } else if (kv1.name == "B" && kv2.name == "B") {
+        return base64_decode(kv1.value).find(base64_decode(kv2.value)) != bytes::npos;
+    } else if (is_set_of(kv1.name, kv2.name)) {
+        for (auto i = kv1.value.Begin(); i != kv1.value.End(); ++i) {
+            if (*i == kv2.value) {
+                return true;
+            }
+        }
+    } else if (kv1.name == "L") {
+        for (auto i = kv1.value.Begin(); i != kv1.value.End(); ++i) {
+            if (!i->IsObject() || i->MemberCount() != 1) {
+                clogger.error("check_CONTAINS received a list whose element is malformed");
+                return false;
+            }
+            const auto& el = *i->MemberBegin();
+            if (el.name == kv2.name && el.value == kv2.value) {
+                return true;
+            }
+        }
+    }
+    return false;
+}
+
+// Check if two JSON-encoded values match with the NOT_CONTAINS relation
+static bool check_NOT_CONTAINS(const rjson::value* v1, const rjson::value& v2) {
+    if (!v1) {
+        return false;
+    }
+    return !check_CONTAINS(v1, v2);
+}
+
 // Check if a JSON-encoded value equals any element of an array, which must have at least one element.
 static bool check_IN(const rjson::value* val, const rjson::value& array) {
     if (!array[0].IsObject() || array[0].MemberCount() != 1) {
@@ -393,10 +449,14 @@ static bool verify_expected_one(const rjson::value& condition, const rjson::valu
         case comparison_operator_type::BETWEEN:
             verify_operand_count(attribute_value_list, exact_size(2), *comparison_operator);
             return check_BETWEEN(got, (*attribute_value_list)[0], (*attribute_value_list)[1]);
-        default:
-            // FIXME: implement all the missing types, so there will be no default here.
-            throw api_error("ValidationException", format("ComparisonOperator {} is not yet supported", *comparison_operator));
+        case comparison_operator_type::CONTAINS:
+            verify_operand_count(attribute_value_list, exact_size(1), *comparison_operator);
+            return check_CONTAINS(got, (*attribute_value_list)[0]);
+        case comparison_operator_type::NOT_CONTAINS:
+            verify_operand_count(attribute_value_list, exact_size(1), *comparison_operator);
+            return check_NOT_CONTAINS(got, (*attribute_value_list)[0]);
         }
+        throw std::logic_error(format("Internal error: corrupted operator enum: {}", int(op)));
     }
 }
 
diff --git a/alternator/conditions.hh b/alternator/conditions.hh
index def0320de0..2aa845b5f6 100644
--- a/alternator/conditions.hh
+++ b/alternator/conditions.hh
@@ -37,7 +37,7 @@
 namespace alternator {
 
 enum class comparison_operator_type {
-    EQ, NE, LE, LT, GE, GT, IN, BETWEEN, CONTAINS, IS_NULL, NOT_NULL, BEGINS_WITH
+    EQ, NE, LE, LT, GE, GT, IN, BETWEEN, CONTAINS, NOT_CONTAINS, IS_NULL, NOT_NULL, BEGINS_WITH
 };
 
 comparison_operator_type get_comparison_operator(const rjson::value& comparison_operator);