diff --git a/CMakeLists.txt b/CMakeLists.txt
index 64155cb..41cf356 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -106,6 +106,7 @@ set(SOURCES
   src/libc/stdlib/llabs.c
   src/libc/stdlib/lldiv.c
   src/libc/stdlib/reallocarray.c
+  src/libc/stdlib/strtoull.c
   # string
   src/libc/string/strchr.c
   src/libc/string/strcpy.c
diff --git a/STATUS b/STATUS
index 55b2090..148bbbd 100644
--- a/STATUS
+++ b/STATUS
@@ -67,6 +67,8 @@ DONE:       Function/symbol/macro is defined, builds, links, and is tested
         files in /usr/share/i18n/locales
      -> Implement setlocale() and localeconv() properly (not hard)
      -> Probably support nl_langinfo(), which is much better than localeconv()
+     -> Fix the "TODO: locale: ..." messages wherever assumptions on the locale
+        are made in the code
 
 7.12 <math.h>
      Provided by Lephenixnoir's port of OpenLibm.
@@ -95,7 +97,11 @@ DONE:       Function/symbol/macro is defined, builds, links, and is tested
      TODO (will give the full list later on)
 
 7.20 <stdlib.h>
-     7.20.1  Numeric conversion functions: TODO
+     7.20.1.1  atof: TODO
+     7.20.1.2  atoi, atol, atoll: TODO
+     7.20.1.3  strtod, strtof, strtold: TODO
+     7.20.1.4  strtol, strtoul, strtoll: TODO
+               strtoull: DONE
      7.20.2  Pseudo-random sequence generation functions: TODO
      7.20.3  Memory management functions: TODO (check existing code first)
      7.20.4  Communication with the environment: TODO
diff --git a/include/stdlib.h b/include/stdlib.h
index cab010d..db5cc60 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -63,4 +63,45 @@ div_t div(int __num, int __denom);
 ldiv_t ldiv(long int __num, long int __denom);
 lldiv_t lldiv(long long int __num, long long int __denom);
 
+/* Numeric conversion functions. */
+
+/* Parse a long int from a string. */
+extern long int strtol(
+	char const * restrict __ptr,
+	char ** restrict __endptr,
+	int __base);
+
+/* Parse a long unsigned int from a string. */
+extern unsigned long int strtoul(
+	char const * restrict __ptr,
+	char ** restrict __endptr,
+	int __base);
+
+/* Parse a long long int from a string. */
+extern long long int strtoll(
+	char const * restrict __ptr,
+	char ** restrict __endptr,
+	int __base);
+
+/* Parse a long long unsigned int from a string. */
+extern unsigned long long int strtoull(
+	char const * restrict __ptr,
+	char ** restrict __endptr,
+	int __base);
+
+/* Parse a double from a string. */
+extern double strtod(
+	char const * restrict __ptr,
+	char ** restrict __endptr);
+
+/* Parse a float from a string. */
+extern float strtof(
+	char const * restrict __ptr,
+	char ** restrict __endptr);
+
+/* Parse a long double from a string. */
+extern long double strtold(
+	char const * restrict __ptr,
+	char ** restrict __endptr);
+
 #endif /*__STDLIB_H__*/
diff --git a/src/libc/stdlib/strtoull.c b/src/libc/stdlib/strtoull.c
new file mode 100644
index 0000000..dcf4ee5
--- /dev/null
+++ b/src/libc/stdlib/strtoull.c
@@ -0,0 +1,62 @@
+#include <stdlib.h>
+#include <stdbool.h>
+#include <ctype.h>
+#include <errno.h>
+
+unsigned long long int strtoull(char const * restrict ptr,
+	char ** restrict endptr, int base)
+{
+	/* Save the value of ptr in endptr now in case the format is invalid */
+	if(endptr) *endptr = (char *)ptr;
+
+	/* Skip initial whitespace */
+	while(isspace(*ptr)) ptr++;
+
+	/* Accept a sign character */
+	bool negative = false;
+	if(*ptr == '-') negative = true;
+	if(*ptr == '-' || *ptr == '+') ptr++;
+
+	unsigned long long x = 0;
+	bool valid = false;
+
+	/* Read prefixes and determine base */
+	if((base == 0 || base == 16) && ptr[0]=='0' && tolower(ptr[1])=='x') {
+		ptr += 2;
+		base = 16;
+	}
+	else if(base == 0 && ptr[0] == '0') {
+		ptr++;
+		base = 8;
+	}
+	else if(base == 0) {
+		base = 10;
+	}
+
+	/* Read digits */
+	while(1) {
+		int v = -1;
+		if(isdigit(*ptr)) v = *ptr - '0';
+		if(islower(*ptr)) v = *ptr - 'a' + 10;
+		if(v == -1 || v >= base) break;
+
+		/* The value is valid as long as there is at least one digit */
+		valid = true;
+
+		/* (x = base*x + v) but with overflow checks */
+		if(__builtin_umulll_overflow(x, base, &x))
+			errno = ERANGE;
+		if(__builtin_uaddll_overflow(x, v, &x))
+			errno = ERANGE;
+
+		ptr++;
+	}
+
+	if(negative) {
+		if(x != 0) errno = ERANGE;
+		x = -x;
+	}
+
+	if(endptr && valid) *endptr = (char *)ptr;
+	return x;
+}