diff --git a/src/accounting/report/reports/balance_sheet.py b/src/accounting/report/reports/balance_sheet.py
index 949ef24..5caeeb1 100644
--- a/src/accounting/report/reports/balance_sheet.py
+++ b/src/accounting/report/reports/balance_sheet.py
@@ -27,6 +27,7 @@ from accounting.locale import gettext
 from accounting.models import Currency, BaseAccount, Account, Transaction, \
     JournalEntry
 from accounting.report.period import Period
+from .utils.base_report import BaseReport
 from .utils.csv_export import BaseCSVRow, csv_download
 from .utils.option_link import OptionLink
 from .utils.page_params import PageParams
@@ -371,7 +372,7 @@ class BalanceSheetPageParams(PageParams):
                 .order_by(Currency.code).all()]
 
 
-class BalanceSheet:
+class BalanceSheet(BaseReport):
     """The balance sheet."""
 
     def __init__(self, currency: Currency, period: Period):
diff --git a/src/accounting/report/reports/income_expenses.py b/src/accounting/report/reports/income_expenses.py
index 23420f8..cf3786e 100644
--- a/src/accounting/report/reports/income_expenses.py
+++ b/src/accounting/report/reports/income_expenses.py
@@ -29,6 +29,7 @@ from accounting.models import Currency, Account, Transaction, JournalEntry
 from accounting.report.income_expense_account import IncomeExpensesAccount
 from accounting.report.period import Period
 from accounting.utils.pagination import Pagination
+from .utils.base_report import BaseReport
 from .utils.csv_export import BaseCSVRow, csv_download
 from .utils.option_link import OptionLink
 from .utils.page_params import PageParams
@@ -387,7 +388,7 @@ def _populate_entries(entries: list[Entry]) -> None:
             entry.account = accounts[entry.entry.account_id]
 
 
-class IncomeExpenses:
+class IncomeExpenses(BaseReport):
     """The income and expenses log."""
 
     def __init__(self, currency: Currency, account: IncomeExpensesAccount,
diff --git a/src/accounting/report/reports/income_statement.py b/src/accounting/report/reports/income_statement.py
index 97697b7..64c8633 100644
--- a/src/accounting/report/reports/income_statement.py
+++ b/src/accounting/report/reports/income_statement.py
@@ -27,6 +27,7 @@ from accounting.locale import gettext
 from accounting.models import Currency, BaseAccount, Account, Transaction, \
     JournalEntry
 from accounting.report.period import Period
+from .utils.base_report import BaseReport
 from .utils.csv_export import BaseCSVRow, csv_download
 from .utils.option_link import OptionLink
 from .utils.page_params import PageParams
@@ -200,7 +201,7 @@ class IncomeStatementPageParams(PageParams):
                 .order_by(Currency.code).all()]
 
 
-class IncomeStatement:
+class IncomeStatement(BaseReport):
     """The income statement."""
 
     def __init__(self, currency: Currency, period: Period):
diff --git a/src/accounting/report/reports/journal.py b/src/accounting/report/reports/journal.py
index 36e9676..5e03186 100644
--- a/src/accounting/report/reports/journal.py
+++ b/src/accounting/report/reports/journal.py
@@ -28,6 +28,7 @@ from accounting.locale import gettext
 from accounting.models import Currency, Account, Transaction, JournalEntry
 from accounting.report.period import Period
 from accounting.utils.pagination import Pagination
+from .utils.base_report import BaseReport
 from .utils.csv_export import BaseCSVRow, csv_download
 from .utils.page_params import PageParams
 from .utils.period_choosers import JournalPeriodChooser
@@ -172,7 +173,7 @@ def _populate_entries(entries: list[Entry]) -> None:
         entry.currency = currencies[entry.entry.currency_code]
 
 
-class Journal:
+class Journal(BaseReport):
     """The journal."""
 
     def __init__(self, period: Period):
diff --git a/src/accounting/report/reports/ledger.py b/src/accounting/report/reports/ledger.py
index 4f6b64d..6e6e659 100644
--- a/src/accounting/report/reports/ledger.py
+++ b/src/accounting/report/reports/ledger.py
@@ -28,6 +28,7 @@ from accounting.locale import gettext
 from accounting.models import Currency, Account, Transaction, JournalEntry
 from accounting.report.period import Period
 from accounting.utils.pagination import Pagination
+from .utils.base_report import BaseReport
 from .utils.csv_export import BaseCSVRow, csv_download
 from .utils.option_link import OptionLink
 from .utils.page_params import PageParams
@@ -342,7 +343,7 @@ def _populate_entries(entries: list[Entry]) -> None:
             entry.note = entry.transaction.note
 
 
-class Ledger:
+class Ledger(BaseReport):
     """The ledger."""
 
     def __init__(self, currency: Currency, account: Account, period: Period):
diff --git a/src/accounting/report/reports/trial_balance.py b/src/accounting/report/reports/trial_balance.py
index d32e006..8c06a63 100644
--- a/src/accounting/report/reports/trial_balance.py
+++ b/src/accounting/report/reports/trial_balance.py
@@ -26,6 +26,7 @@ from accounting import db
 from accounting.locale import gettext
 from accounting.models import Currency, Account, Transaction, JournalEntry
 from accounting.report.period import Period
+from .utils.base_report import BaseReport
 from .utils.csv_export import BaseCSVRow, csv_download
 from .utils.option_link import OptionLink
 from .utils.page_params import PageParams
@@ -162,7 +163,7 @@ class TrialBalancePageParams(PageParams):
                 .order_by(Currency.code).all()]
 
 
-class TrialBalance:
+class TrialBalance(BaseReport):
     """The trial balance."""
 
     def __init__(self, currency: Currency, period: Period):
diff --git a/src/accounting/report/reports/utils/base_report.py b/src/accounting/report/reports/utils/base_report.py
new file mode 100644
index 0000000..9438c87
--- /dev/null
+++ b/src/accounting/report/reports/utils/base_report.py
@@ -0,0 +1,40 @@
+# The Mia! Accounting Flask Project.
+# Author: imacat@mail.imacat.idv.tw (imacat), 2023/3/8
+
+#  Copyright (c) 2023 imacat.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+"""The base report.
+
+"""
+from abc import ABC, abstractmethod
+
+from flask import Response
+
+
+class BaseReport(ABC):
+    """The base report class."""
+
+    @abstractmethod
+    def csv(self) -> Response:
+        """Returns the report as CSV for download.
+
+        :return: The response of the report for download.
+        """
+
+    @abstractmethod
+    def html(self) -> str:
+        """Composes and returns the report as HTML.
+
+        :return: The report as HTML.
+        """