diff --git a/migrations/20240709145933_create_idempotency_table.sql b/migrations/20240709145933_create_idempotency_table.sql
new file mode 100644
index 0000000..9d5de22
--- /dev/null
+++ b/migrations/20240709145933_create_idempotency_table.sql
@@ -0,0 +1,15 @@
+-- Add migration script here
+CREATE TYPE header_pair AS (
+    name TEXT,
+    value BYTEA
+);
+
+CREATE TABLE idempotency (
+    user_id uuid NOT NULL REFERENCES users(user_id),
+    idempotency_key TEXT NOT NULL,
+    response_status_code SMALLINT NOT NULL,
+    response_headers header_pair[] NOT NULL,
+    response_body BYTEA NOT NULL,
+    created_at timestamptz NOT NULL,
+    PRIMARY KEY(user_id, idempotency_key)
+);
\ No newline at end of file
diff --git a/src/idempotency/key.rs b/src/idempotency/key.rs
new file mode 100644
index 0000000..8bf10ff
--- /dev/null
+++ b/src/idempotency/key.rs
@@ -0,0 +1,32 @@
+#[derive(Debug)]
+pub struct IdempotencyKey(String);
+
+impl TryFrom<String> for IdempotencyKey {
+    type Error = anyhow::Error;
+
+    fn try_from(s: String) -> Result<Self, Self::Error> {
+        if s.is_empty() {
+            anyhow::bail!("The idempotency key cannot be empty");
+        }
+        let max_length = 50;
+        if s.len() >= max_length {
+            anyhow::bail!(
+                "The idempotency key must be shorter
+                    than {max_length} characters"
+            );
+        }
+        Ok(Self(s))
+    }
+}
+
+impl From<IdempotencyKey> for String {
+    fn from(k: IdempotencyKey) -> Self {
+        k.0
+    }
+}
+
+impl AsRef<str> for IdempotencyKey {
+    fn as_ref(&self) -> &str {
+        &self.0
+    }
+}
diff --git a/src/idempotency/mod.rs b/src/idempotency/mod.rs
new file mode 100644
index 0000000..bda225f
--- /dev/null
+++ b/src/idempotency/mod.rs
@@ -0,0 +1,5 @@
+mod key;
+mod persistence;
+
+pub use key::IdempotencyKey;
+pub use persistence::{get_saved_response, save_response};
diff --git a/src/idempotency/persistence.rs b/src/idempotency/persistence.rs
new file mode 100644
index 0000000..945dbba
--- /dev/null
+++ b/src/idempotency/persistence.rs
@@ -0,0 +1,97 @@
+use super::IdempotencyKey;
+use actix_web::body::to_bytes;
+use actix_web::http::StatusCode;
+use actix_web::HttpResponse;
+use sqlx::postgres::PgHasArrayType;
+use sqlx::PgPool;
+use uuid::Uuid;
+
+#[derive(Debug, sqlx::Type)]
+#[sqlx(type_name = "header_pair")]
+struct HeaderPairRecord {
+    name: String,
+    value: Vec<u8>,
+}
+
+pub async fn get_saved_response(
+    pool: &PgPool,
+    idempotency_key: &IdempotencyKey,
+    user_id: Uuid,
+) -> Result<Option<HttpResponse>, anyhow::Error> {
+    let saved_response = sqlx::query!(
+        r#"
+            SELECT
+                response_status_code,
+                response_headers as "response_headers: Vec<HeaderPairRecord>",
+                response_body
+            FROM idempotency
+            WHERE
+                user_id = $1 AND
+                idempotency_key = $2
+        "#,
+        user_id,
+        idempotency_key.as_ref()
+    )
+    .fetch_optional(pool)
+    .await?;
+
+    if let Some(r) = saved_response {
+        let status_code = StatusCode::from_u16(r.response_status_code.try_into()?)?;
+        let mut response = HttpResponse::build(status_code);
+        for HeaderPairRecord { name, value } in r.response_headers {
+            response.append_header((name, value));
+        }
+        Ok(Some(response.body(r.response_body)))
+    } else {
+        Ok(None)
+    }
+}
+
+impl PgHasArrayType for HeaderPairRecord {
+    fn array_type_info() -> sqlx::postgres::PgTypeInfo {
+        sqlx::postgres::PgTypeInfo::with_name("_header_pair")
+    }
+}
+
+pub async fn save_response(
+    pool: &PgPool,
+    idempotency_key: &IdempotencyKey,
+    user_id: Uuid,
+    http_response: HttpResponse,
+) -> Result<HttpResponse, anyhow::Error> {
+    let (response_head, body) = http_response.into_parts();
+    let body = to_bytes(body).await.map_err(|e| anyhow::anyhow!("{}", e))?;
+    let status_code = response_head.status().as_u16() as i16;
+    let headers = {
+        let mut h = Vec::with_capacity(response_head.headers().len());
+        for (name, value) in response_head.headers().iter() {
+            let name = name.as_str().to_owned();
+            let value = value.as_bytes().to_owned();
+            h.push(HeaderPairRecord { name, value });
+        }
+        h
+    };
+
+    sqlx::query_unchecked!(
+        r#"
+            INSERT INTO idempotency (
+                user_id,
+                idempotency_key,
+                response_status_code,
+                response_headers,
+                response_body,
+                created_at)
+            VALUES ($1, $2, $3, $4, $5, now())
+        "#,
+        user_id,
+        idempotency_key.as_ref(),
+        status_code,
+        headers,
+        body.as_ref()
+    )
+    .execute(pool)
+    .await?;
+
+    let http_response = response_head.set_body(body).map_into_boxed_body();
+    Ok(http_response)
+}
diff --git a/src/lib.rs b/src/lib.rs
index f469562..67bf1d0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,6 +2,7 @@ pub mod authentication;
 pub mod configuration;
 pub mod domain;
 pub mod email_client;
+pub mod idempotency;
 pub mod routes;
 pub mod session_state;
 pub mod startup;
diff --git a/src/routes/admin/newsletters/get.rs b/src/routes/admin/newsletters/get.rs
index 6731b0b..148bfe5 100644
--- a/src/routes/admin/newsletters/get.rs
+++ b/src/routes/admin/newsletters/get.rs
@@ -11,6 +11,7 @@ pub async fn publish_newsletter_form(
         writeln!(msg_html, "<p><i>{}</i></p>", m.content()).unwrap();
     }
 
+    let idempotency_key = uuid::Uuid::new_v4();
     Ok(HttpResponse::Ok()
         .content_type(ContentType::html())
         .body(format!(
@@ -49,6 +50,7 @@ pub async fn publish_newsletter_form(
                             ></textarea>
                         </label>
                         <br>
+                        <input hidden type="text" name="idempotency_key" value="{idempotency_key}">
                         <button type="submit">Publish</button>
                     </form>
                     <p><a href="/admin/dashboard">&lt;- Back</a></p>
diff --git a/src/routes/admin/newsletters/post.rs b/src/routes/admin/newsletters/post.rs
index eb766eb..9f35811 100644
--- a/src/routes/admin/newsletters/post.rs
+++ b/src/routes/admin/newsletters/post.rs
@@ -1,6 +1,8 @@
 use crate::authentication::UserId;
 use crate::domain::SubscriberEmail;
 use crate::email_client::EmailClient;
+use crate::idempotency::{get_saved_response, save_response, IdempotencyKey};
+use crate::utils::e400;
 use crate::utils::{e500, see_other};
 use actix_web::web::ReqData;
 use actix_web::{web, HttpResponse};
@@ -13,6 +15,7 @@ pub struct FormData {
     title: String,
     text_content: String,
     html_content: String,
+    idempotency_key: String,
 }
 
 #[tracing::instrument(
@@ -22,21 +25,32 @@ pub struct FormData {
 )]
 pub async fn publish_newsletter(
     form: web::Form<FormData>,
-    user_id: ReqData<UserId>,
     pool: web::Data<PgPool>,
     email_client: web::Data<EmailClient>,
+    user_id: ReqData<UserId>,
 ) -> Result<HttpResponse, actix_web::Error> {
+    let user_id = user_id.into_inner();
+    let FormData {
+        title,
+        text_content,
+        html_content,
+        idempotency_key,
+    } = form.0;
+    let idempotency_key: IdempotencyKey = idempotency_key.try_into().map_err(e400)?;
+
+    if let Some(saved_response) = get_saved_response(&pool, &idempotency_key, *user_id)
+        .await
+        .map_err(e500)?
+    {
+        return Ok(saved_response);
+    }
+
     let subscribers = get_confirmed_subscribers(&pool).await.map_err(e500)?;
     for subscriber in subscribers {
         match subscriber {
             Ok(subscriber) => {
                 email_client
-                    .send_email(
-                        &subscriber.email,
-                        &form.title,
-                        &form.html_content,
-                        &form.text_content,
-                    )
+                    .send_email(&subscriber.email, &title, &html_content, &text_content)
                     .await
                     .with_context(|| {
                         format!("Failed to send newsletter issue to {}", subscriber.email)
@@ -53,7 +67,12 @@ pub async fn publish_newsletter(
         }
     }
     FlashMessage::info("The newsletter issue has been published!").send();
-    Ok(see_other("/admin/newsletters"))
+    let response = see_other("/admin/newsletters");
+    let response = save_response(&pool, &idempotency_key, *user_id, response)
+        .await
+        .map_err(e500)?;
+
+    Ok(response)
 }
 
 struct ConfirmedSubscriber {
diff --git a/src/utils.rs b/src/utils.rs
index 08c76ab..819c97e 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -1,7 +1,6 @@
 use actix_web::http::header::LOCATION;
 use actix_web::HttpResponse;
 
-// Return an opaque 500 while preserving the error root's cause for logging.
 pub fn e500<T>(e: T) -> actix_web::Error
 where
     T: std::fmt::Debug + std::fmt::Display + 'static,
@@ -9,6 +8,13 @@ where
     actix_web::error::ErrorInternalServerError(e)
 }
 
+pub fn e400<T: std::fmt::Debug + std::fmt::Display>(e: T) -> actix_web::Error
+where
+    T: std::fmt::Debug + std::fmt::Display + 'static,
+{
+    actix_web::error::ErrorBadRequest(e)
+}
+
 pub fn see_other(location: &str) -> HttpResponse {
     HttpResponse::SeeOther()
         .insert_header((LOCATION, location))
diff --git a/tests/api/newsletter.rs b/tests/api/newsletter.rs
index 4a134d3..b6faae8 100644
--- a/tests/api/newsletter.rs
+++ b/tests/api/newsletter.rs
@@ -54,6 +54,7 @@ async fn newsletters_are_not_delivered_to_unconfirmed_subscribers() {
         "title": "Newsletter title",
         "text_content": "Newsletter body as plain text",
         "html_content": "<p>Newsletter body as HTML</p>",
+        "idempotency_key": uuid::Uuid::new_v4().to_string()
     });
     let response = app.post_publish_newsletter(&newsletter_request_body).await;
     assert_is_redirect_to(&response, "/admin/newsletters");
@@ -83,6 +84,7 @@ async fn newsletters_are_delivered_to_confirmed_subscribers() {
         "title": "Newsletter title",
         "text_content": "Newsletter body as plain text",
         "html_content": "<p>Newsletter body as HTML</p>",
+        "idempotency_key": uuid::Uuid::new_v4().to_string()
     });
     let response = app.post_publish_newsletter(&newsletter_request_body).await;
     assert_is_redirect_to(&response, "/admin/newsletters");
@@ -115,6 +117,7 @@ async fn you_must_be_logged_in_to_publish_a_newsletter() {
         "title": "Newsletter title",
         "text_content": "Newsletter body as plain text",
         "html_content": "<p>Newsletter body as HTML</p>",
+        "idempotency_key": uuid::Uuid::new_v4().to_string()
     });
     let response = app.post_publish_newsletter(&newsletter_request_body).await;
 
@@ -142,8 +145,6 @@ async fn newsletter_creation_is_idempotent() {
         "title": "Newsletter title",
         "text_content": "Newsletter body as plain text",
         "html_content": "<p>Newsletter body as HTML</p>",
-        // We expect the idempotency key as part of the
-        // form data, not as an header
         "idempotency_key": uuid::Uuid::new_v4().to_string()
     });
     let response = app.post_publish_newsletter(&newsletter_request_body).await;
@@ -159,6 +160,7 @@ async fn newsletter_creation_is_idempotent() {
 
     // Act - Part 4 - Follow the redirect
     let html_page = app.get_publish_newsletter_html().await;
+    dbg!(&html_page);
     assert!(html_page.contains("<p><i>The newsletter issue has been published!</i></p>"));
 
     // Mock verifies on Drop that we have sent the newsletter email **once**