diff --git a/0-lang0py/lang0py.py b/0-lang0py/lang0py.py
index f990edb..089888f 100644
--- a/0-lang0py/lang0py.py
+++ b/0-lang0py/lang0py.py
@@ -166,6 +166,53 @@ def parsestatreturn(indent):
     emit(eol)
     skipchar(eol)
 
+def parsestatcheck(indent):
+    skipchar(' ')
+    emit('    ' * indent)
+    emit('assert ')
+
+    func_name = lexident()
+    emit(func_name)
+    emit('(')
+    notfirst = False
+
+    while True:
+        skipchar(' ')
+
+        if ':' == peek():
+            break
+
+        if notfirst:
+            emit(', ')
+
+        if quote == peek():
+            parseconststring()
+        else:
+            parseexprvarref()
+
+        notfirst = True
+
+    skipchar(':')
+
+    emit('), (')
+    notfirst = False
+
+    while True:
+        skipchar(' ')
+
+        if notfirst:
+            emit(', ')
+
+        if quote == peek():
+            parseconststring()
+        else:
+            parseexprvarref()
+
+        if eol == peek():
+            break
+        notfirst = True
+    emitln(')')
+
 def parsestattrace(indent):
     skipchar(' ')
     emit('    ' * indent)
@@ -211,6 +258,10 @@ def parsestat(indent):
         parsestatreturn(indent)
         return
 
+    if call == "check":
+        parsestatcheck(indent)
+        return
+
     if call == "trace":
         parsestattrace(indent)
         return
diff --git a/1-lang0py/lang0py.lang0 b/1-lang0py/lang0py.lang0
index 929b837..0628b7b 100644
--- a/1-lang0py/lang0py.lang0
+++ b/1-lang0py/lang0py.lang0
@@ -157,6 +157,82 @@ parsestatreturn indent:
 	skipchar eol
 /
 
+parsestatcheck indent:
+	declare char
+	declare funcname
+	declare iscolon
+	declare isnotquote
+	declare isquote
+	declare notfirst
+
+	skipchar " "
+	emit indent
+	emit "assert "
+
+	calc funcname lexident
+	emit funcname
+	emit "("
+
+	set notfirst ""
+	forever
+		skipchar " "
+		calc char peek
+		calc iscolon eq char ":"
+		if iscolon
+			break
+		/
+
+		if notfirst
+			emit ", "
+		/
+
+		calc char peek
+		calc isquote eq char quote
+		calc isnotquote not isquote
+		if isquote
+			parseconststring
+		/
+		if isnotquote
+			parseexprvarref
+		/
+
+		set notfirst "1"
+	/
+
+	skipchar ":"
+
+	emit "), ("
+	set notfirst ""
+
+	forever
+		skipchar " "
+
+		if notfirst
+			emit ", "
+		/
+
+		calc char peek
+		calc isquote eq char quote
+		calc isnotquote not isquote
+		if isquote
+			parseconststring
+		/
+		if isnotquote
+			parseexprvarref
+		/
+
+		calc char peek
+		calc iseol eq char eol
+		if iseol
+			break
+		/
+
+		set notfirst "1"
+	/
+
+	emitln ")"
+/
+
 parsestattrace indent:
 	emit indent
 	emit "trace("
@@ -214,6 +290,11 @@ parsestat indent:
 		parsestattrace indent
 		return
 	/
+	calc ischeck eq call "check"
+	if ischeck
+		parsestatcheck indent
+		return
+	/
 	emit indent
 	emit call
 	emit "("
diff --git a/2-lang0c/lang0c.lang0 b/2-lang0c/lang0c.lang0
index fdf9287..7b6cbca 100644
--- a/2-lang0c/lang0c.lang0
+++ b/2-lang0c/lang0c.lang0
@@ -214,6 +214,88 @@ parsestatreturn indent:
 	return
 /
 
+parsestatcheck indent:
+	declare char
+	declare funcname
+	declare iscolon
+	declare iseol
+	declare isnotquote
+	declare isquote
+	declare notfirst
+
+	skipchar " "
+	emit indent
+	emit "if( 0 == strlen("
+
+	calc funcname lexident
+	emit funcname
+	emit "("
+
+	set notfirst ""
+	forever
+		skipchar " "
+		calc char peek
+		calc iscolon eq char ":"
+		if iscolon
+			break
+		/
+
+		if notfirst
+			emit ", "
+		/
+
+		calc char peek
+		calc isquote eq char quote
+		calc isnotquote not isquote
+		if isquote
+			parseconststring
+		/
+		if isnotquote
+			parseexprvarref
+		/
+
+		set notfirst "1"
+	/
+
+	skipchar ":"
+
+	emitln ")) )"
+	emit indent
+	emitln "{"
+
+	forever
+		skipchar " "
+
+		emit indent
+		emit "    fprintf(stderr, "
+		emit quote
+		emit "%s "
+		emit quote
+		emit ", "
+		calc char peek
+		calc isquote eq char quote
+		calc isnotquote not isquote
+		if isquote
+			parseconststring
+		/
+		if isnotquote
+			parseexprvarref
+		/
+		emitln ");"
+
+		calc char peek
+		calc iseol eq char eol
+		if iseol
+			break
+		/
+	/
+
+	emit indent
+	emitln "    exit(1);"
+	emit indent
+	emitln "}"
+/
+
 parsestattrace indent:
 	declare varname
 	emit indent
@@ -278,6 +360,11 @@ parsestat indent:
 		parsestatreturn indent
 		return
 	/
+	calc iscall eq call "check"
+	if iscall
+		parsestatcheck indent
+		return
+	/
 	calc iscall eq call "trace"
 	if iscall
 		parsestattrace indent
diff --git a/README.md b/README.md
index d14e8ea..082ac23 100644
--- a/README.md
+++ b/README.md
@@ -113,6 +113,10 @@ Returns the given value. You can not give an argument if your function is never
 
 ### Builtins
 
+#### check func [arg0..] : msg [arg0..]
+
+Calls the given function with the given arguments. If the function's return value is not found to be true, outputs the given `msg` and halts the program.
+
 #### trace
 
 Writes the name and value of the variable passed to stderr if the TRACE environment variable is set.
diff --git a/tests/Makefile b/tests/Makefile
index 510228f..030c87c 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -8,23 +8,27 @@ PYPREFIX=/usr
 CYTHON=cython3
 CC=gcc
 
-TESTLIST=$(shell ls *.lang0 | sed 's/.lang0//')
+# builtincheckfalse is separate since it's supposed to crash. It's a test for the test 'framework', if you will.
+TESTLIST=$(shell ls *.lang0 | grep -v 'builtincheckfalse' | sed 's/.lang0//')
 
 all: check
 
 check: all-it0.results all-it1.results all-it2.results
 	! grep -v 'Success' $^
 
-all-it0.results: $(addprefix build/,$(addsuffix .it0, $(TESTLIST)))
+all-it0.results: $(addprefix build/,$(addsuffix .it0, $(TESTLIST))) build/builtincheckfalse.it0
 	-rm -f $@
+	echo "Hello" | build/builtincheckfalse.it0 2> /dev/null 2>&1 | grep 'Success'
 	$(foreach test,$(TESTLIST), echo -n "$(test)   " >> $@; echo "Hello" | ./build/$(test).it0 >> $@ ; echo "" >> $@ ;)
 
-all-it1.results: $(addprefix build/,$(addsuffix .it1, $(TESTLIST)))
+all-it1.results: $(addprefix build/,$(addsuffix .it1, $(TESTLIST))) build/builtincheckfalse.it0
 	-rm -f $@
+	echo "Hello" | build/builtincheckfalse.it0 2> /dev/null 2>&1 | grep 'Success'
 	$(foreach test,$(TESTLIST), echo -n "$(test)   " >> $@; echo "Hello" | ./build/$(test).it1 >> $@ ; echo "" >> $@ ;)
 
-all-it2.results: $(addprefix build/,$(addsuffix .it2, $(TESTLIST)))
+all-it2.results: $(addprefix build/,$(addsuffix .it2, $(TESTLIST))) build/builtincheckfalse.it0
 	-rm -f $@
+	echo "Hello" | build/builtincheckfalse.it0 2> /dev/null 2>&1 | grep 'Success'
 	$(foreach test,$(TESTLIST), echo -n "$(test)   " >> $@; echo "Hello" | ./build/$(test).it2 >> $@ ; echo "" >> $@ ;)
 
 clean:
diff --git a/tests/builtincheckfalse.lang0 b/tests/builtincheckfalse.lang0
new file mode 100644
index 0000000..a433392
--- /dev/null
+++ b/tests/builtincheckfalse.lang0
@@ -0,0 +1,3 @@
+main:
+	check not "1" : "Success"
+/
diff --git a/tests/builtinchecktrue.lang0 b/tests/builtinchecktrue.lang0
new file mode 100644
index 0000000..3566241
--- /dev/null
+++ b/tests/builtinchecktrue.lang0
@@ -0,0 +1,4 @@
+main:
+	check not "" : "Failure"
+	emit "Success"
+/
diff --git a/tests/stdlibaddstringchar.lang0 b/tests/stdlibaddstringchar.lang0
index 5d83c80..991f778 100644
--- a/tests/stdlibaddstringchar.lang0
+++ b/tests/stdlibaddstringchar.lang0
@@ -1,5 +1,6 @@
 main:
 	declare result
-	calc result addstringchar "Succes" "s"
-	emit result
+	calc result addstringchar "abc" "d"
+	check eq result "abcd" : "Adding abc and d should be abcd"
+	emit "Success"
 /
diff --git a/tests/stdlibeqfalse.lang0 b/tests/stdlibeqfalse.lang0
index e2e52d3..5c66e3c 100644
--- a/tests/stdlibeqfalse.lang0
+++ b/tests/stdlibeqfalse.lang0
@@ -1,8 +1,6 @@
 main:
 	declare bool
 	calc bool eq "1" "2"
-	if bool
-		return
-	/
+	check not bool : "1 should not equal 2"
 	emit "Success"
 /
diff --git a/tests/stdlibeqtrue.lang0 b/tests/stdlibeqtrue.lang0
index dd44ca0..f495e10 100644
--- a/tests/stdlibeqtrue.lang0
+++ b/tests/stdlibeqtrue.lang0
@@ -1,7 +1,5 @@
 main:
 	declare bool
-	calc bool eq "1" "1"
-	if bool
-		emit "Success"
-	/
+	check eq "1" "1" : "1 should equal 1"
+	emit "Success"
 /
diff --git a/tests/stdlibltfalse.lang0 b/tests/stdlibltfalse.lang0
index 0a7ad58..4963cfc 100644
--- a/tests/stdlibltfalse.lang0
+++ b/tests/stdlibltfalse.lang0
@@ -1,8 +1,6 @@
 main:
 	declare bool
 	calc bool lt "b" "a"
-	if bool
-		return
-	/
+	check not bool : "a should should sort before b"
 	emit "Success"
 /
diff --git a/tests/stdliblttrue.lang0 b/tests/stdliblttrue.lang0
index f3f3433..8c3ca69 100644
--- a/tests/stdliblttrue.lang0
+++ b/tests/stdliblttrue.lang0
@@ -1,7 +1,5 @@
 main:
 	declare bool
-	calc bool lt "a" "b"
-	if bool
-		emit "Success"
-	/
+	check lt "a" "b" : "a should should sort before b"
+	emit "Success"
 /
diff --git a/tests/stdlibpeek.lang0 b/tests/stdlibpeek.lang0
index 9455c7b..713f261 100644
--- a/tests/stdlibpeek.lang0
+++ b/tests/stdlibpeek.lang0
@@ -1,9 +1,6 @@
 main:
 	declare char
-	declare ish
 	calc char peek
-	calc ish eq char "H"
-	if ish
-		emit "Success"
-	/
+	check eq char "H" : "The first byte we're passed is expected to be an 'H'"
+	emit "Success"
 /
diff --git a/tests/stdlibskip.lang0 b/tests/stdlibskip.lang0
index 1392458..e6e4ae0 100644
--- a/tests/stdlibskip.lang0
+++ b/tests/stdlibskip.lang0
@@ -1,24 +1,12 @@
 main:
 	declare char
-	declare ish
-	declare iso
-	declare notish
-	declare notiso
 	skip
 	calc char peek
-	calc ish eq char "e"
-	calc notish not ish
-	if notish
-		return
-	/
+	check eq char "e" : "The second byte we're passed is expected to be an 'e'"
 	skip
 	skip
 	skip
 	calc char peek
-	calc iso eq char "o"
-	calc notiso not iso
-	if notish
-		return
-	/
+	check eq char "o" : "The fifth byte we're passed is expected to be an 'o'"
 	emit "Success"
 /