Interpersonal connectedness (IC) is an essential social function across life but is critical for young children. IC begins at birth and is vital to emerging physiological and affect regulation, attachment, communication, and cognition. IC occurs through dynamic interactive behavioral coordination, resulting in an embodied dyadic state of shared affect, comfort, and rapport. IC is a poorly defined construct in early childhood but incorporates the bidirectional coordination of shared social timing, unconscious mimicry, and mutual gaze. Only recently have researchers examined IC in atypically developing children. Young autistic children struggle with dynamic coordination of interactive behaviors, affecting the downstream products of IC. Therefore, IC is a vital target for autism intervention. Consequently, the assessment of IC is clinically relevant as an outcome measure. Current measures of parent-child interactions have limited clinical use because of the complexity of video coding, time involved, and training needed. The Connection Coder (CC) is a new, easy-to-use smart-device app for the clinical evaluation of IC. The CC allows clinicians to efficiently record and code a 5-minute naturalistic interaction within any setting. This study measured the duration, quality, and continuity of mother-child connectedness in 3 to 5-year-old neurotypical and autistic children to test the validity and reliability of the CC. The CC-Average Degree of Connection had good concurrent validity with a highly regarded measure of interactive behavior (CIB; Feldman, 1998) and good convergent validity with the Parenting Stress Index-SF (Abidin, 2012). All items of the CC had moderate to strong correlations with the CARS-2 (Schopler et al., 2010). [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]