diff --git a/README.md b/README.md
index 5561e65..60aca37 100644
--- a/README.md
+++ b/README.md
@@ -1 +1,31 @@
 This repository contains an ESLint plugin for validating usage of [`18next`](https://github.com/i18next/i18next).
+
+## Installation
+
+```
+yarn add -D @lifeomic/eslint-plugin-18next
+```
+
+## Usage
+
+```javascript
+// .eslintrc.js
+module.exports = {
+  plugins: ['@lifeomic/i18next'],
+  rules: {
+    'i18next/default-value': [
+      'error',
+      {
+        /* optional options object */
+      },
+    ],
+  },
+};
+```
+
+## Rule Options
+
+#### `default-value`
+
+- `translateFunctionNames`: an array of translation function names to validate. Default is `['t']`
+- `allowKeyOnly`: whether to allow e.g. `t('just-the-key')`
diff --git a/src/default-value/default-value.test.ts b/src/default-value/default-value.test.ts
new file mode 100644
index 0000000..c8fde28
--- /dev/null
+++ b/src/default-value/default-value.test.ts
@@ -0,0 +1,104 @@
+import { RuleTester } from 'eslint';
+import { defaultValueRule, Errors } from './default-value';
+
+const tester = new RuleTester();
+
+tester.run('default-value', defaultValueRule, {
+  valid: [
+    {
+      code: `
+        t('key', { defaultValue: 'some long value' })
+      `,
+    },
+    {
+      code: `
+        i18n.t('key', { defaultValue: 'some long value' })
+      `,
+    },
+    {
+      code: `
+        translate('key', { defaultValue: 'some long value' })
+      `,
+      options: [{ translateFunctionNames: ['translate'] }],
+    },
+    {
+      code: `
+        translate('key', { defaultValue: 'some {{long}} value' })
+      `,
+      options: [{ translateFunctionNames: ['notTranslate'] }],
+    },
+    {
+      code: `
+        t('key', {
+          defaultValue: 'some {{long}} value',
+          long: 'something'
+        })
+      `,
+    },
+    {
+      code: `
+        t('key')
+      `,
+      options: [{ allowKeyOnly: true }],
+    },
+    {
+      code: `
+        someOtherFunction()
+      `,
+    },
+  ],
+  invalid: [
+    {
+      code: `
+        t('key')
+      `,
+      errors: [Errors.twoArguments],
+    },
+    {
+      code: `
+        t('key', 'bogus')
+      `,
+      errors: [Errors.optionsObject],
+    },
+    {
+      code: `
+        t('key', { notDefaultValue: 'something' })
+      `,
+      errors: [Errors.defaultValuePresence],
+    },
+    {
+      code: `
+        t('key', { defaultValue: true })
+      `,
+      errors: [Errors.defaultValuePresence],
+    },
+    {
+      code: `
+        t('key', { defaultValue: 'some {{long}} value' })
+      `,
+      errors: [Errors.missingVariable('long')],
+    },
+    {
+      code: `
+        translate('key', {
+          defaultValue: 'some {{long }} value'
+        })
+      `,
+      options: [{ translateFunctionNames: ['translate'] }],
+      errors: [Errors.missingVariable('long')],
+    },
+    {
+      code: `
+        translate('key', {
+          defaultValue: 'some {{long}} value'
+        });
+
+        t('key', {
+          defaultValue: 'some {{short}} value'
+        })
+      `,
+      options: [{ translateFunctionNames: ['translate', 't'] }],
+      errors: [Errors.missingVariable('long'), Errors.missingVariable('short')],
+    },
+  ],
+});
diff --git a/src/default-value/default-value.ts b/src/default-value/default-value.ts
new file mode 100644
index 0000000..e42f981
--- /dev/null
+++ b/src/default-value/default-value.ts
@@ -0,0 +1,124 @@
+import { Rule } from 'eslint';
+import { ObjectExpression } from 'estree';
+import { findPropertyOnNode, isFunctionCallExpression } from '../util';
+
+export const Errors = {
+  twoArguments: 'Translate function should have two arguments',
+  optionsObject: 'Translate function options should be an object',
+  defaultValuePresence:
+    'Translate function defaultValue must a string property on the second argument',
+  missingVariable: (named: string) =>
+    `Missing "${named}" in translate variables`,
+};
+
+const findDefaultValueOnObject = (node: ObjectExpression) => {
+  const prop = findPropertyOnNode(node, { named: 'defaultValue' });
+
+  if (prop?.value.type === 'Literal' && typeof prop.value.value === 'string') {
+    const variableNames = prop.value.value
+      .match(/{{[ a-zA-Z0-9]+}}/g)
+      ?.map((value) => value.replace('{{', '').replace('}}', '').trim());
+
+    return {
+      value: prop.value.value,
+      variableNames: variableNames ?? [],
+    };
+  }
+};
+
+export type DefaultValueOptions = {
+  translateFunctionNames: string[];
+  allowKeyOnly: boolean;
+};
+
+const parseOptions = (context: Rule.RuleContext): DefaultValueOptions => {
+  const DEFAULT: DefaultValueOptions = {
+    translateFunctionNames: ['t'],
+    allowKeyOnly: false,
+  };
+
+  if (!context.options.length) {
+    return DEFAULT;
+  }
+
+  return {
+    ...DEFAULT,
+    ...context.options[0],
+  };
+};
+
+export const defaultValueRule: Rule.RuleModule = {
+  meta: {
+    schema: [
+      {
+        type: 'object',
+        additionalProperties: true,
+        properties: {
+          translateFunctionNames: {
+            type: 'array',
+            items: { type: 'string' },
+          },
+          allowKeyOnly: {
+            type: 'boolean',
+          },
+        },
+      },
+    ],
+  },
+  create: (context) => {
+    const options = parseOptions(context);
+
+    return {
+      CallExpression: (node) => {
+        if (
+          !isFunctionCallExpression(node, {
+            named: options.translateFunctionNames,
+          })
+        ) {
+          return;
+        }
+
+        if (node.arguments.length < 2) {
+          if (options.allowKeyOnly && node.arguments.length === 1) {
+            return;
+          }
+          context.report({
+            node,
+            message: Errors.twoArguments,
+          });
+          return;
+        }
+
+        const secondArg = node.arguments[1];
+
+        if (secondArg.type !== 'ObjectExpression') {
+          context.report({
+            node,
+            message: Errors.optionsObject,
+          });
+          return;
+        }
+
+        const defaultValue = findDefaultValueOnObject(secondArg);
+        if (!defaultValue) {
+          context.report({
+            node,
+            message: Errors.defaultValuePresence,
+          });
+          return;
+        }
+
+        for (const variable of defaultValue.variableNames) {
+          const prop = findPropertyOnNode(secondArg, { named: variable });
+          if (!prop) {
+            context.report({
+              node,
+              message: Errors.missingVariable(variable),
+            });
+            return;
+          }
+        }
+      },
+    };
+  },
+};
diff --git a/src/default-value/index.ts b/src/default-value/index.ts
new file mode 100644
index 0000000..32ca480
--- /dev/null
+++ b/src/default-value/index.ts
@@ -0,0 +1 @@
+export * from './default-value';
diff --git a/src/index.test.ts b/src/index.test.ts
deleted file mode 100644
index 51b6e4c..0000000
--- a/src/index.test.ts
+++ /dev/null
@@ -1,5 +0,0 @@
-import { placeholder } from './';
-
-test('placeholder', () => {
-  expect(placeholder).toBe('placeholder');
-});
diff --git a/src/index.ts b/src/index.ts
index dd98f81..f43537e 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1 +1,7 @@
-export const placeholder = 'placeholder';
+import { defaultValueRule } from './default-value/default-value';
+
+export default {
+  rules: {
+    'default-value': defaultValueRule,
+  },
+};
diff --git a/src/util.ts b/src/util.ts
new file mode 100644
index 0000000..ed029f4
--- /dev/null
+++ b/src/util.ts
@@ -0,0 +1,30 @@
+import { CallExpression, ObjectExpression } from 'estree';
+
+export const isFunctionCallExpression = (
+  node: CallExpression,
+  options: { named: string[] },
+) => {
+  return (
+    (node.callee.type === 'MemberExpression' &&
+      options.named.includes((node.callee.property as any).name)) ||
+    (node.callee.type === 'Identifier' &&
+      options.named.includes(node.callee.name))
+  );
+};
+
+export const findPropertyOnNode = (
+  { properties }: ObjectExpression,
+  options: {
+    named: string;
+  },
+) => {
+  for (const prop of properties) {
+    if (
+      prop.type === 'Property' &&
+      prop.key.type === 'Identifier' &&
+      prop.key.name === options.named
+    ) {
+      return prop;
+    }
+  }
+};