diff --git a/OpenRA.Game/FieldLoader.cs b/OpenRA.Game/FieldLoader.cs
index 21806ea98e..52273e288a 100644
--- a/OpenRA.Game/FieldLoader.cs
+++ b/OpenRA.Game/FieldLoader.cs
@@ -24,6 +24,26 @@ namespace OpenRA
 {
 	public static class FieldLoader
 	{
+		public class MissingFieldsException : YamlException
+		{
+			public readonly string[] Missing;
+			public readonly string Header;
+			public override string Message
+			{
+				get
+				{
+					return (string.IsNullOrEmpty(Header) ? "" : Header + ": ") + Missing[0]
+						+ string.Concat(Missing.Skip(1).Select(m => ", " + m));
+				}
+			}
+
+			public MissingFieldsException(string[] missing, string header = null, string headerSingle = null) : base(null)
+			{
+				Header = missing.Length > 1 ? header : headerSingle ?? header;
+				Missing = missing;
+			}
+		}
+
 		public static Func<string, Type, string, object> InvalidValueAction = (s, t, f) =>
 		{
 			throw new InvalidOperationException("FieldLoader: Cannot parse `{0}` into `{1}.{2}` ".F(s, f, t));
@@ -45,6 +65,7 @@ namespace OpenRA
 		public static void Load(object self, MiniYaml my)
 		{
 			var loadInfo = TypeLoadInfo[self.GetType()];
+			var missing = new List<string>();
 
 			Dictionary<string, MiniYaml> md = null;
 
@@ -52,19 +73,33 @@ namespace OpenRA
 			{
 				object val;
 
+				if (md == null)
+					md = my.ToDictionary();
 				if (fli.Loader != null)
-					val = fli.Loader(my);
+				{
+					if (!fli.Attribute.Required || md.ContainsKey(fli.YamlName))
+						val = fli.Loader(my);
+					else
+					{
+						missing.Add(fli.YamlName);
+						continue;
+					}
+				}
 				else
 				{
-					if (md == null)
-						md = my.ToDictionary();
-
 					if (!TryGetValueFromYaml(fli.YamlName, fli.Field, md, out val))
+					{
+						if (fli.Attribute.Required)
+							missing.Add(fli.YamlName);
 						continue;
+					}
 				}
 
 				fli.Field.SetValue(self, val);
 			}
+
+			if (missing.Any())
+				throw new MissingFieldsException(missing.ToArray());
 		}
 
 		static bool TryGetValueFromYaml(string yamlName, FieldInfo field, Dictionary<string, MiniYaml> md, out object ret)
@@ -557,12 +592,20 @@ namespace OpenRA
 				: base(false) { }
 		}
 
+		[AttributeUsage(AttributeTargets.Field)]
+		public sealed class RequireAttribute : SerializeAttribute
+		{
+			public RequireAttribute()
+				: base(true, true) { }
+		}
+
 		[AttributeUsage(AttributeTargets.Field)]
 		public sealed class LoadUsingAttribute : SerializeAttribute
 		{
-			public LoadUsingAttribute(string loader)
+			public LoadUsingAttribute(string loader, bool required = false)
 			{
 				Loader = loader;
+				Required = required;
 			}
 		}
 
@@ -577,10 +620,12 @@ namespace OpenRA
 			public string YamlName;
 			public string Loader;
 			public bool FromYamlKey;
+			public bool Required;
 
-			public SerializeAttribute(bool serialize = true)
+			public SerializeAttribute(bool serialize = true, bool required = false)
 			{
 				Serialize = serialize;
+				Required = required;
 			}
 
 			internal Func<MiniYaml, object> GetLoader(Type type)
diff --git a/OpenRA.Game/GameRules/ActorInfo.cs b/OpenRA.Game/GameRules/ActorInfo.cs
index 65d2060c00..ee0ecbb7b4 100644
--- a/OpenRA.Game/GameRules/ActorInfo.cs
+++ b/OpenRA.Game/GameRules/ActorInfo.cs
@@ -94,7 +94,16 @@ namespace OpenRA
 				throw new YamlException("Junk value `{0}` on trait node {1}"
 				.F(my.Value, traitName));
 			var info = Game.CreateObject<ITraitInfo>(traitName + "Info");
-			FieldLoader.Load(info, my);
+			try
+			{
+				FieldLoader.Load(info, my);
+			}
+			catch (FieldLoader.MissingFieldsException e)
+			{
+				var header = "Trait name " + traitName + ": " + (e.Missing.Length > 1 ? "Required properties missing" : "Required property missing");
+				throw new FieldLoader.MissingFieldsException(e.Missing, header);
+			}
+
 			return info;
 		}
 
diff --git a/OpenRA.Mods.Common/Traits/GainsExperience.cs b/OpenRA.Mods.Common/Traits/GainsExperience.cs
index 8644671a4d..c76e5ad808 100644
--- a/OpenRA.Mods.Common/Traits/GainsExperience.cs
+++ b/OpenRA.Mods.Common/Traits/GainsExperience.cs
@@ -20,8 +20,8 @@ namespace OpenRA.Mods.Common.Traits
 	[Desc("This actor's experience increases when it has killed a GivesExperience actor.")]
 	public class GainsExperienceInfo : ITraitInfo, Requires<ValuedInfo>, Requires<UpgradeManagerInfo>
 	{
-		[FieldLoader.LoadUsing("LoadUpgrades")]
-		[Desc("Upgrades to grant at each level. (Required property)",
+		[FieldLoader.LoadUsing("LoadUpgrades", true)]
+		[Desc("Upgrades to grant at each level.",
 			"Key is the XP requirements for each level as a percentage of our own value.",
 			"Value is a list of the upgrade types to grant")]
 		public readonly Dictionary<int, string[]> Upgrades = null;
@@ -36,10 +36,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		static object LoadUpgrades(MiniYaml y)
 		{
-			MiniYaml upgrades;
-
-			if (!y.ToDictionary().TryGetValue("Upgrades", out upgrades))
-				throw new YamlException("GainsExperience is missing Upgrades.");
+			MiniYaml upgrades = y.ToDictionary()["Upgrades"];
 
 			return upgrades.Nodes.ToDictionary(
 				kv => FieldLoader.GetValue<int>("(key)", kv.Key),
diff --git a/OpenRA.Mods.Common/UtilityCommands/ExtractTraitDocsCommand.cs b/OpenRA.Mods.Common/UtilityCommands/ExtractTraitDocsCommand.cs
index faf688bcab..df4e2fd00d 100644
--- a/OpenRA.Mods.Common/UtilityCommands/ExtractTraitDocsCommand.cs
+++ b/OpenRA.Mods.Common/UtilityCommands/ExtractTraitDocsCommand.cs
@@ -86,7 +86,8 @@ namespace OpenRA.Mods.Common.UtilityCommands
 				{
 					var fieldDescLines = info.Field.GetCustomAttributes<DescAttribute>(true).SelectMany(d => d.Lines);
 					var fieldType = FriendlyTypeName(info.Field.FieldType);
-					var defaultValue = FieldSaver.SaveField(liveTraitInfo, info.Field.Name).Value.Value;
+					var loadInfo = info.Field.GetCustomAttributes<FieldLoader.SerializeAttribute>(true).FirstOrDefault();
+					var defaultValue = loadInfo != null && loadInfo.Required ? "<em>(required)</em>" : FieldSaver.SaveField(liveTraitInfo, info.Field.Name).Value.Value;
 					doc.Append("<tr><td>{0}</td><td>{1}</td><td>{2}</td>".F(info.YamlName, defaultValue, fieldType));
 					doc.Append("<td>");
 					foreach (var line in fieldDescLines)