diff --git a/src/main.rs b/src/main.rs
index 8a1a54e..9e99ed4 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -42,6 +42,7 @@ fn main() {
                 .takes_value(true)
                 .value_name("FILE"),
         )
+        .arg(Arg::with_name("warning").help("Show warnings").short("w"))
         .get_matches();
 
     // We can unwrap because config is required.
@@ -70,7 +71,7 @@ fn main() {
 
     config.update(profile);
 
-    config.print();
+    config.print(matches.is_present("warning"));
 }
 
 // Read the content of a file and return it as String.
diff --git a/src/parsers/slice.rs b/src/parsers/slice.rs
index bf64a20..6186342 100644
--- a/src/parsers/slice.rs
+++ b/src/parsers/slice.rs
@@ -4,11 +4,15 @@
 // license that can be found in the LICENSE file.
 
 use std::collections::BTreeMap;
+use std::vec::Vec;
 
+use colored::Colorize;
 use indexmap::IndexMap;
 use pest::error::Error;
 use pest::Parser;
 
+const SEPARATOR: &str = "=";
+
 #[derive(Parser)]
 #[grammar = "grammars/slice.pest"]
 struct SliceParser;
@@ -16,6 +20,7 @@ struct SliceParser;
 /// zeroc configuration file.
 pub struct Config {
     map: IndexMap<String, String>,
+    warnings: Vec<String>,
 }
 
 /// Create Config object from a string.
@@ -24,6 +29,7 @@ pub fn parse(s: &String) -> Result<Config, Error<Rule>> {
 
     let mut config = Config {
         map: IndexMap::new(),
+        warnings: Vec::new(),
     };
 
     for line in parsed.into_inner() {
@@ -44,18 +50,34 @@ pub fn parse(s: &String) -> Result<Config, Error<Rule>> {
 
 impl Config {
     /// Overwrite Config keys using the ones from map.
+    ///
+    /// If a key is only in the profile, it will not be added to the
+    /// configuration file, and a warning will be saved.
     pub fn update(&mut self, map: BTreeMap<String, String>) {
         for (k, v) in map {
             if let Some(val) = self.map.get_mut(&k) {
                 *val = v;
+            } else {
+                self.warnings.push(format!(
+                    "{}: {} key is not present in config file.",
+                    "Warning".yellow(),
+                    k
+                ));
             }
         }
     }
 
     /// Print Config to stdout.
-    pub fn print(&self) {
+    ///
+    /// If show_warnings is true, warnings will be printed to stderr.
+    pub fn print(&self, show_warnings: bool) {
+        if show_warnings {
+            for warning in &self.warnings {
+                eprintln!("{}", warning);
+            }
+        }
         for (k, v) in &self.map {
-            println!("{} = {}", k, v);
+            println!("{} {} {}", k, SEPARATOR, v);
         }
     }
 }
@@ -83,13 +105,18 @@ fn update() {
     "
     .to_string();
 
-    let mut m = BTreeMap::new();
-    m.insert("Author.like".to_string(), "rust, python, c++".to_string());
-    m.insert("Author.web".to_string(), "example.org:8080".to_string());
+    let mut profile = BTreeMap::new();
+    profile.insert("Author.like".to_string(), "rust, python, c++".to_string());
+    profile.insert("Author.web".to_string(), "example.org:8080".to_string());
+    profile.insert("Only.here".to_string(), "value".to_string());
 
     let mut c = parse(&conf).unwrap();
-    c.update(m);
+    c.update(profile);
 
     assert_eq!(c.map["Author.like"], "rust, python, c++");
     assert_eq!(c.map["Author.web"], "example.org:8080");
+
+    assert_eq!(c.warnings.len(), 1);
+    let m: Vec<&str> = c.warnings.first().unwrap().split(":").collect();
+    assert_eq!(m[1].trim(), "Only.here key is not present in config file.");
 }