diff --git a/website/source/docs/providers/aws/r/elb.html.markdown b/website/source/docs/providers/aws/r/elb.html.markdown
index f851a5552..31e1eb448 100644
--- a/website/source/docs/providers/aws/r/elb.html.markdown
+++ b/website/source/docs/providers/aws/r/elb.html.markdown
@@ -51,15 +51,18 @@ resource "aws_elb" "bar" {
 The following arguments are supported:
 
 * `name` - (Required) The name of the ELB
-* `availability_zones` - (Optional) The AZ's to serve traffic in.
+* `availability_zones` - (Required for an EC2-classic ELB) The AZ's to serve traffic in.
 * `security_groups` - (Optional) A list of security group IDs to assign to the ELB.
-* `subnets` - (Optional) A list of subnets to attach to the ELB.
+* `subnets` - (Required for a VPC ELB) A list of subnet IDs to attach to the ELB.
 * `instances` - (Optional) A list of instance ids to place in the ELB pool.
 * `internal` - (Optional) If true, ELB will be an internal ELB.
 * `listener` - (Required) A list of listener blocks. Listeners documented below.
 * `health_check` - (Optional) A health_check block. Health Check documented below.
 * `cross_zone_load_balancing` - (Optional) Enable cross-zone load balancing.
 
+Exactly one of `availability_zones` or `subnets` must be specified: this
+determines if the ELB exists in a VPC or in EC2-classic.
+
 Listeners support the following:
 
 * `instance_port` - (Required) The port on the instance to route to