"Arriving in a splash of fanfare, with a cast headed by Joan Crawford, I Saw What You Did is a generally broad and belabored expansion of a nifty idea...Tightened, minus about half an hour and with the entire story held to the impressionable viewpoint of the youngsters, the first picture would have brightened and chilled considerably more. Unfortunately, William Castle, the director-producer, dawdles the tempo. And there is a redundant middle chapter involving the aroused, snarling killer...The youngsters, Andi Garrett, Sarah Lane and a perky little toddler named Sharyl Locke, are altogether delightful."
- Howard Thompson, The New York Times

"Though the plot is thin, the film offers a chilling atmosphere and the performances are effective"
TV Guide

"...pretty mild stuff by Castle standards, coming off such outrageous horror fests as The Tingler [1959] and Homicidal [1961], but the results are still great fun and unmistakably display the touch of the master...The shower sequence is one of the more amusing Psycho [1960] imitations; in this case, the victim is fully dressed, outside the shower, and wielding the knife. Figure that one out! It also climaxes in a nice bit of pre-Argento glass-shattering that really seems strong for a 1965 film aimed at adolescents."
- Nathaniel Thompson, Mondo Digital

"It's a hokey version of Rear Window [1954], complete with all of Castle's usual parlor tricks. This is one terror tale that is strangely still suitable for all ages. Lots of fun."
- Christopher Null, www.filmcritic.com

"A few chilling moments, but it all depends on your susceptibility."
- David Pirie, TimeOut Film Guide

"Tense, gimmick-free Castle shocker..."
- Leonard Maltin's Movie Guide