It's not technically on the spectrum, but, more often than not, people who are on the spectrum can also have it. I've always thought of it like one of those "if a is b, and b is c, then a must also be c" type of problems, except a doesn't always have to be c, YK? SPD can be a comorbid disorder, meaning that it exists alongside another disorder, such as an ASD, ADD/ADHD, MR, even gifted kids can have SPD (of course, kids on the spectrum can also be gifted, but that's for another thread.) However, it can also stand alone. I've heard talk of them adding SPD to the DSM V under ASDs but I'm not sure if they should. It doesn't seem like a spectrum disorder to me. It seems like something that can coexist with them, but not something that can stand alone as a spectrum disorder. I hope that answered your question.