diff --git a/scrapy/commands/__init__.py b/scrapy/commands/__init__.py
index a573a03d9..9f8e6986a 100644
--- a/scrapy/commands/__init__.py
+++ b/scrapy/commands/__init__.py
@@ -23,7 +23,8 @@ class ScrapyCommand:
         self.settings = None  # set in scrapy.cmdline
 
     def set_crawler(self, crawler):
-        assert not hasattr(self, '_crawler'), "crawler already set"
+        if hasattr(self, '_crawler'):
+            raise RuntimeError("crawler already set")
         self._crawler = crawler
 
     def syntax(self):
diff --git a/scrapy/contracts/default.py b/scrapy/contracts/default.py
index 3002fc702..a1b0f8f22 100644
--- a/scrapy/contracts/default.py
+++ b/scrapy/contracts/default.py
@@ -58,7 +58,11 @@ class ReturnsContract(Contract):
     def __init__(self, *args, **kwargs):
         super(ReturnsContract, self).__init__(*args, **kwargs)
 
-        assert len(self.args) in [1, 2, 3]
+        if len(self.args) not in [1, 2, 3]:
+            raise ValueError(
+                "Incorrect argument quantity: expected 1, 2 or 3, got %i"
+                % len(self.args)
+            )
         self.obj_name = self.args[0] or None
         self.obj_type = self.objects[self.obj_name]
 
diff --git a/scrapy/core/downloader/middleware.py b/scrapy/core/downloader/middleware.py
index 5a03dcdf7..4c2eea522 100644
--- a/scrapy/core/downloader/middleware.py
+++ b/scrapy/core/downloader/middleware.py
@@ -45,8 +45,9 @@ class DownloaderMiddlewareManager(MiddlewareManager):
 
         @defer.inlineCallbacks
         def process_response(response):
-            assert response is not None, 'Received None in process_response'
-            if isinstance(response, Request):
+            if response is None:
+                raise TypeError("Received None in process_response")
+            elif isinstance(response, Request):
                 return response
 
             for method in self.methods['process_response']:
diff --git a/scrapy/core/engine.py b/scrapy/core/engine.py
index 66cf9ad9a..77d71846e 100644
--- a/scrapy/core/engine.py
+++ b/scrapy/core/engine.py
@@ -73,7 +73,8 @@ class ExecutionEngine:
     @defer.inlineCallbacks
     def start(self):
         """Start the execution engine"""
-        assert not self.running, "Engine already running"
+        if self.running:
+            raise RuntimeError("Engine already running")
         self.start_time = time()
         yield self.signals.send_catch_log_deferred(signal=signals.engine_started)
         self.running = True
@@ -82,7 +83,8 @@ class ExecutionEngine:
 
     def stop(self):
         """Stop the execution engine gracefully"""
-        assert self.running, "Engine not running"
+        if not self.running:
+            raise RuntimeError("Engine not running")
         self.running = False
         dfd = self._close_all_spiders()
         return dfd.addBoth(lambda _: self._finish_stopping_engine())
@@ -165,7 +167,11 @@ class ExecutionEngine:
         return d
 
     def _handle_downloader_output(self, response, request, spider):
-        assert isinstance(response, (Request, Response, Failure)), response
+        if not isinstance(response, (Request, Response, Failure)):
+            raise TypeError(
+                "Incorrect type: expected Request, Response or Failure, got %s: %r"
+                % (type(response), response)
+            )
         # downloader middleware can return requests (for example, redirects)
         if isinstance(response, Request):
             self.crawl(response, spider)
@@ -205,8 +211,8 @@ class ExecutionEngine:
         return not bool(self.slot)
 
     def crawl(self, request, spider):
-        assert spider in self.open_spiders, \
-            "Spider %r not opened when crawling: %s" % (spider.name, request)
+        if spider not in self.open_spiders:
+            raise RuntimeError("Spider %r not opened when crawling: %s" % (spider.name, request))
         self.schedule(request, spider)
         self.slot.nextcall.schedule()
 
@@ -232,7 +238,11 @@ class ExecutionEngine:
         slot.add_request(request)
 
         def _on_success(response):
-            assert isinstance(response, (Response, Request))
+            if not isinstance(response, (Response, Request)):
+                raise TypeError(
+                    "Incorrect type: expected Response or Request, got %s: %r"
+                    % (type(response), response)
+                )
             if isinstance(response, Response):
                 response.request = request  # tie request to response received
                 logkws = self.logformatter.crawled(request, response, spider)
@@ -253,8 +263,8 @@ class ExecutionEngine:
 
     @defer.inlineCallbacks
     def open_spider(self, spider, start_requests=(), close_if_idle=True):
-        assert self.has_capacity(), "No free spider slot when opening %r" % \
-            spider.name
+        if not self.has_capacity():
+            raise RuntimeError("No free spider slot when opening %r" % spider.name)
         logger.info("Spider opened", extra={'spider': spider})
         nextcall = CallLaterOnce(self._next_request, spider)
         scheduler = self.scheduler_cls.from_crawler(self.crawler)
diff --git a/scrapy/core/scraper.py b/scrapy/core/scraper.py
index 3e4826216..edbb4dd66 100644
--- a/scrapy/core/scraper.py
+++ b/scrapy/core/scraper.py
@@ -123,7 +123,11 @@ class Scraper:
     def _scrape(self, response, request, spider):
         """Handle the downloaded response or failure through the spider
         callback/errback"""
-        assert isinstance(response, (Response, Failure))
+        if not isinstance(response, (Response, Failure)):
+            raise TypeError(
+                "Incorrect type: expected Response or Failure, got %s: %r"
+                % (type(response), response)
+            )
 
         dfd = self._scrape2(response, request, spider)  # returns spider's processed output
         dfd.addErrback(self.handle_spider_error, request, response, spider)
diff --git a/scrapy/crawler.py b/scrapy/crawler.py
index 20990ea41..6f43771e2 100644
--- a/scrapy/crawler.py
+++ b/scrapy/crawler.py
@@ -78,7 +78,8 @@ class Crawler:
 
     @defer.inlineCallbacks
     def crawl(self, *args, **kwargs):
-        assert not self.crawling, "Crawling already taking place"
+        if self.crawling:
+            raise RuntimeError("Crawling already taking place")
         self.crawling = True
 
         try:
diff --git a/scrapy/http/request/__init__.py b/scrapy/http/request/__init__.py
index 0a6637af8..a98ba9960 100644
--- a/scrapy/http/request/__init__.py
+++ b/scrapy/http/request/__init__.py
@@ -24,7 +24,8 @@ class Request(object_ref):
         self.method = str(method).upper()
         self._set_url(url)
         self._set_body(body)
-        assert isinstance(priority, int), "Request priority not an integer: %r" % priority
+        if not isinstance(priority, int):
+            raise TypeError("Request priority not an integer: %r" % priority)
         self.priority = priority
 
         if callback is not None and not callable(callback):
diff --git a/scrapy/pipelines/files.py b/scrapy/pipelines/files.py
index aab645d3d..ae365db5b 100644
--- a/scrapy/pipelines/files.py
+++ b/scrapy/pipelines/files.py
@@ -106,7 +106,8 @@ class S3FilesStore:
         else:
             from boto.s3.connection import S3Connection
             self.S3Connection = S3Connection
-        assert uri.startswith('s3://')
+        if not uri.startswith("s3://"):
+            raise ValueError("Incorrect URI scheme in %s, expected 's3'" % uri)
         self.bucket, self.prefix = uri[5:].split('/', 1)
 
     def stat_file(self, path, info):
@@ -266,7 +267,8 @@ class FTPFilesStore:
     USE_ACTIVE_MODE = None
 
     def __init__(self, uri):
-        assert uri.startswith('ftp://')
+        if not uri.startswith("ftp://"):
+            raise ValueError("Incorrect URI scheme in %s, expected 'ftp'" % uri)
         u = urlparse(uri)
         self.port = u.port
         self.host = u.hostname
diff --git a/scrapy/utils/iterators.py b/scrapy/utils/iterators.py
index b71419111..5e15bf0c8 100644
--- a/scrapy/utils/iterators.py
+++ b/scrapy/utils/iterators.py
@@ -128,10 +128,12 @@ def csviter(obj, delimiter=None, headers=None, encoding=None, quotechar=None):
 
 def _body_or_str(obj, unicode=True):
     expected_types = (Response, str, bytes)
-    assert isinstance(obj, expected_types), \
-        "obj must be %s, not %s" % (
-            " or ".join(t.__name__ for t in expected_types),
-            type(obj).__name__)
+    if not isinstance(obj, expected_types):
+        expected_types_str = " or ".join(t.__name__ for t in expected_types)
+        raise TypeError(
+            "Object %r must be %s, not %s"
+            % (obj, expected_types_str, type(obj).__name__)
+        )
     if isinstance(obj, Response):
         if not unicode:
             return obj.body
diff --git a/scrapy/utils/reactor.py b/scrapy/utils/reactor.py
index 5308812d6..3c705f69b 100644
--- a/scrapy/utils/reactor.py
+++ b/scrapy/utils/reactor.py
@@ -9,7 +9,8 @@ from scrapy.utils.misc import load_object
 def listen_tcp(portrange, host, factory):
     """Like reactor.listenTCP but tries different ports in a range."""
     from twisted.internet import reactor
-    assert len(portrange) <= 2, "invalid portrange: %s" % portrange
+    if len(portrange) > 2:
+        raise ValueError("invalid portrange: %s" % portrange)
     if not portrange:
         return reactor.listenTCP(0, factory, interface=host)
     if not hasattr(portrange, '__iter__'):
diff --git a/tests/test_utils_iterators.py b/tests/test_utils_iterators.py
index ec8311298..a85087619 100644
--- a/tests/test_utils_iterators.py
+++ b/tests/test_utils_iterators.py
@@ -157,7 +157,7 @@ class XmliterTestCase(unittest.TestCase):
 
     def test_xmliter_objtype_exception(self):
         i = self.xmliter(42, 'product')
-        self.assertRaises(AssertionError, next, i)
+        self.assertRaises(TypeError, next, i)
 
     def test_xmliter_encoding(self):
         body = b'<?xml version="1.0" encoding="ISO-8859-9"?>\n<xml>\n    <item>Some Turkish Characters \xd6\xc7\xde\xdd\xd0\xdc \xfc\xf0\xfd\xfe\xe7\xf6</item>\n</xml>\n\n'