diff --git a/Step_0.md b/Step_0.md
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+# Step-0: define a service
+
+This section presents an example of a very simple service definition that
+receives a message from a remote client. The messages contains the users's
+name and sends's back a greeting for that person.
+
+Here it is in full; to be used to generate gRPC code it's defined in it's own
+file [helloworld.proto](helloworld.proto).
+
+```
+syntax = "proto3";
+
+package helloworld;
+
+// The request message containing the user's name.
+message HelloRequest {
+  optional string name = 1;
+}
+
+// The response message containing the greetings
+message HelloReply {
+  optional string message = 1;
+}
+
+// The greeting service definition.
+service Greeting {
+
+  // Sends a greeting
+  rpc hello (HelloRequest) returns (HelloReply) {
+  }
+}
+
+```
+
+The service stanza of the messages is an example of protobuf service IDL
+(Interface Defintion Language).  Here, it defines a very simple service that
+receives a request and returns a response.
+
+Next in [Step-1](Step-1.md), we'll use protoc to generate code this simple
+definition.
diff --git a/helloworld.proto b/helloworld.proto
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+++ b/helloworld.proto
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+syntax = "proto3";
+
+package helloworld;
+
+// The request message containing the user's name.
+message HelloRequest {
+  optional string name = 1;
+}
+
+// The response message containing the greetings
+message HelloReply {
+  optional string message = 1;
+}
+
+// The greeting service definition.
+service Greeting {
+  // Sends a greeting
+  rpc hello (HelloRequest) returns (HelloReply) {
+  }
+}