diff --git a/README.md b/README.md
index 84b8f57..223bcd8 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,7 @@ Further options to add to the commands above:
 - Add `--strict` to enable strict mode in validation for schemas and numbers (as defined by [ajv](https://ajv.js.org/strict-mode.html) for options `strictSchema`, `strictNumbers` and `strictTuples`)
 - To lint local JSON files: `--lint` (add `--verbose` to get a diff with the changes required)
 - To format / pretty-print local JSON files: `--format` (Attention: this will override the source files without warning!)
+- To run custom validation code: `--custom ./path/to/validation.js` - The validation.js needs to contain a class that implements the `BaseValidator` interface. See [custom.example.js](./custom.example.js) for an example.
 
 **Note on API support:** Validating lists of STAC items/collections (i.e. `GET /collections` and `GET /collections/:id/items`) is partially supported.
 It only checks the contained items/collections, but not the other parts of the response (e.g. `links`).
@@ -74,7 +75,8 @@ The schema map is an object instead of string separated with a `=` character.
   "lint": true,
   "format": false,
   "strict": true,
-	"all": false
+	"all": false,
+	"custom": null
 }
 ```
 
diff --git a/custom.example.js b/custom.example.js
new file mode 100644
index 0000000..426da3d
--- /dev/null
+++ b/custom.example.js
@@ -0,0 +1,24 @@
+const BaseValidator = require('./src/baseValidator.js');
+const assert = require('assert');
+
+class CustomValidator extends BaseValidator {
+
+	/**
+	 * Any custom validation routines you want to run.
+	 * 
+	 * You can either create a list of errors or just throw on the first error (e.g. using `assert` functions).
+	 * 
+	 * @param {STAC} data
+	 * @param {import('.').Report} report
+	 * @param {import('.').Config} config
+	 * @throws {Error}
+	 * @returns {Array.<Error>}
+	 */
+	async afterValidation(data, report, config) {
+		// assert.deepEqual(data.example, [1,2,3]);
+		return [];
+	}
+
+}
+
+module.exports = CustomValidator;
diff --git a/package.json b/package.json
index 135c03d..884f251 100644
--- a/package.json
+++ b/package.json
@@ -32,6 +32,7 @@
     "fs-extra": "^10.0.0",
     "jest-diff": "^29.0.1",
     "klaw": "^4.0.1",
+    "stac-js": "^0.0.8",
     "yargs": "^17.7.2"
   },
   "devDependencies": {
diff --git a/src/baseValidator.js b/src/baseValidator.js
new file mode 100644
index 0000000..804baad
--- /dev/null
+++ b/src/baseValidator.js
@@ -0,0 +1,40 @@
+const { STAC } = import('stac-js');
+
+class BaseValidator {
+
+	/**
+	 * 
+	 */
+	constructor() {
+	}
+
+	/**
+	 * Any preprocessing work you want to do on the data.
+	 * 
+	 * @param {Object} data
+	 * @param {import('.').Report} report
+	 * @param {import('.').Config} config
+	 * @returns {Object}
+	 */
+	async afterLoading(data, report, config) {
+		return data;
+	}
+
+	/**
+	 * Any custom validation routines you want to run.
+	 * 
+	 * You can either create a list of errors or just throw on the first error (e.g. using `assert` functions).
+	 * 
+	 * @param {STAC} data
+	 * @param {import('.').Report} report
+	 * @param {import('.').Config} config
+	 * @throws {Error}
+	 * @returns {Array.<Error>}
+	 */
+	async afterValidation(data, report, config) {
+		return [];
+	}
+
+}
+
+module.exports = BaseValidator;
diff --git a/src/cli.js b/src/cli.js
index 3db0c2a..5e16b26 100644
--- a/src/cli.js
+++ b/src/cli.js
@@ -1,4 +1,5 @@
 const fs = require('fs-extra');
+const path = require('path');
 const { version } = require('../package.json');
 const ConfigSource = require('./config.js');
 const validate = require('../src/index.js');
@@ -71,6 +72,12 @@ async function run() {
 		}
 	}
 
+	if (config.custom) {
+		const absPath = path.resolve(process.cwd(), config.custom);
+		const validator = require(absPath);
+		config.customValidator = new validator();
+	}
+
 	// Finally run validation
 	const result = await validate(data, config);
 
diff --git a/src/config.js b/src/config.js
index c6c997d..9a0fafa 100644
--- a/src/config.js
+++ b/src/config.js
@@ -37,6 +37,11 @@ function fromCLI() {
     description: 'Validate against a specific local schema (e.g. an external extension). Provide the schema URI and the local path separated by an equal sign.\nExample: https://stac-extensions.github.io/foobar/v1.0.0/schema.json=./json-schema/schema.json',
 		coerce: strArrayToObject
   })
+	.option('custom', {
+    type: 'string',
+		default: null,
+    description: 'Load a custom validation routine from a JavaScript file.'
+	})
   .option('ignoreCerts', {
     type: 'boolean',
 		default: false,
diff --git a/src/index.js b/src/index.js
index 3d2e18c..b8f82d3 100644
--- a/src/index.js
+++ b/src/index.js
@@ -2,6 +2,7 @@ const versions = require('compare-versions');
 
 const { createAjv, isUrl, loadSchemaFromUri, normalizePath, isObject } = require('./utils');
 const defaultLoader = require('./loader/default');
+const BaseValidator = require('./baseValidator');
 
 /**
  * @typedef Config
@@ -10,6 +11,7 @@ const defaultLoader = require('./loader/default');
  * @property {string|null} [schemas=null] Validate against schemas in a local or remote STAC folder.
  * @property {Object.<string, string>} [schemaMap={}] Validate against a specific local schema (e.g. an external extension). Provide the schema URI as key and the local path as value.
  * @property {boolean} [strict=false] Enable strict mode in validation for schemas and numbers (as defined by ajv for options `strictSchema`, `strictNumbers` and `strictTuples
+ * @property {BaseValidator} [customValidator=null] A validator with custom rules.
  */
 
 /**
@@ -20,12 +22,19 @@ const defaultLoader = require('./loader/default');
  * @property {string} version
  * @property {boolean} valid
  * @property {Array.<string>} messages
- * @property {Array.<*>} results
  * @property {Array.<Report>} children
- * @property {Extensions.<Object>} extensions
+ * @property {Results} results
  * @property {boolean} apiList
  */
 
+/**
+ * @typedef Results
+ * @type {Object}
+ * @property {OArray.<Error>} core
+ * @property {Object.<string, Array.<Error>>} extensions
+ * @property {Array.<Error>} custom
+ */
+
 /**
  * @returns {Report}
  */
@@ -40,7 +49,8 @@ function createReport() {
 		children: [],
 		results: {
 			core: [],
-			extensions: {}
+			extensions: {},
+			custom: []
 		},
 		apiList: false
 	};
@@ -129,6 +139,10 @@ async function validateOne(source, config, report = null) {
 	report.version = data.stac_version;
 	report.type = data.type;
 
+	if (config.customValidator) {
+		data = await config.customValidator.afterLoading(data, report, config);
+	}
+
 	if (typeof config.lintFn === 'function') {
 		report = await config.lintFn(source, report, config);
 	}
@@ -181,6 +195,22 @@ async function validateOne(source, config, report = null) {
 		await validateSchema('extensions', schema, data, report, config);
 	}
 
+	if (config.customValidator) {
+		const { default: create } = await import('stac-js');
+		const stac = create(data, false, false);
+		try {
+			report.results.custom = await config.customValidator.afterValidation(stac, report, config);
+		} catch (error) {
+			report.results.custom = [
+				error
+			];
+		} finally {
+			if (report.results.custom.length > 0) {
+				report.valid = false;
+			}
+		}
+	}
+
 	return report;
 }
 
diff --git a/src/nodeUtils.js b/src/nodeUtils.js
index 040edab..dc866cf 100644
--- a/src/nodeUtils.js
+++ b/src/nodeUtils.js
@@ -2,7 +2,7 @@ const klaw = require('klaw');
 const fs = require('fs-extra');
 const path = require('path');
 
-const { isUrl } = require('./utils');
+const { isUrl, isObject } = require('./utils');
 
 const SCHEMA_CHOICE = ['anyOf', 'oneOf'];
 
@@ -109,6 +109,9 @@ function printReport(report, config) {
 				console.info("Extensions: None");
 			}
 		}
+		if (config.custom) {
+			printAjvValidationResult(report.results.custom, 'Custom', report.valid, config);
+		}
 	}
 
 	report.children.forEach(child => printReport(child, config));
@@ -172,7 +175,7 @@ function isSchemaChoice(schemaPath) {
 
 function makeAjvErrorMessage(error) {
 	let message = error.message;
-	if (Object.keys(error.params).length > 0) {
+	if (isObject(error.params) && Object.keys(error.params).length > 0) {
 		let params = Object.entries(error.params)
 			.map(([key, value]) => {
 				let label = key.replace(/([^A-Z]+)([A-Z])/g, "$1 $2").toLowerCase();