diff --git a/g3doc/style-guide.md b/g3doc/style-guide.md
index 71d1196df2..b32163f906 100644
--- a/g3doc/style-guide.md
+++ b/g3doc/style-guide.md
@@ -127,6 +127,19 @@ For example,
 See the [source code for `rtc::ArrayView`](api/array_view.h) for more detailed
 docs.
 
+### Strings
+
+WebRTC uses std::string, with content assumed to be UTF-8. Note that this
+has to be verified whenever accepting external input.
+
+For concatenation of strings, use rtc::SimpleStringBuilder.
+
+The following string building tools are NOT recommended:
+* The + operator. See https://abseil.io/tips/3 for why not.
+* absl::StrCat, absl::StrAppend, absl::StrJoin. These are optimized for
+  speed, not code size, and have significant code size overhead.
+* strcat. It is too easy to create buffer overflows.
+
 ### sigslot
 
 SIGSLOT IS DEPRECATED.