diff --git a/balancer/balancer.go b/balancer/balancer.go
index 499acbd9..729be8d0 100644
--- a/balancer/balancer.go
+++ b/balancer/balancer.go
@@ -36,9 +36,12 @@ var (
 	m = make(map[string]Builder)
 )
 
-// Register registers the balancer builder to the balancer map.
-// b.Name (lowercased) will be used as the name registered with
-// this builder.
+// Register registers the balancer builder to the balancer map. b.Name
+// (lowercased) will be used as the name registered with this builder.
+//
+// NOTE: this function must only be called during initialization time (i.e. in
+// an init() function), and is not thread-safe. If multiple Balancers are
+// registered with the same name, the one registered last will take effect.
 func Register(b Builder) {
 	m[strings.ToLower(b.Name())] = b
 }
diff --git a/resolver/resolver.go b/resolver/resolver.go
index 775ee4d0..6b90dbab 100644
--- a/resolver/resolver.go
+++ b/resolver/resolver.go
@@ -29,8 +29,12 @@ var (
 
 // TODO(bar) install dns resolver in init(){}.
 
-// Register registers the resolver builder to the resolver map.
-// b.Scheme will be used as the scheme registered with this builder.
+// Register registers the resolver builder to the resolver map. b.Scheme will be
+// used as the scheme registered with this builder.
+//
+// NOTE: this function must only be called during initialization time (i.e. in
+// an init() function), and is not thread-safe. If multiple Resolvers are
+// registered with the same name, the one registered last will take effect.
 func Register(b Builder) {
 	m[b.Scheme()] = b
 }