diff --git a/SPEC.md b/SPEC.md
index cfb65cf..c26f7be 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -242,6 +242,13 @@ distinction in KDL between real numbers, integers, and floating point numbers.
 It's up to individual implementations to determine how to represent KDL
 numbers.
 
+Implementations MUST support number values representable as IEEE754 floating
+point numbers. That is, all implementations must support at least the numbers
+between `-(2^53 - 1)` and `2^53 - 1`. Larger or smaller values MAY error, or
+be represented as `Infinity`, or `NaN`, or any other value of the
+implementation's choosing. If you want to reliably represent numbers outside
+of this range, use the `string` type to represent them.
+
 There are four syntaxes for Numbers: Decimal, Hexadecimal, Octal, and Binary.
 
 * All numbers may optionally start with one of `-` or `+`, which determine whether they'll be positive or negative.