This dissertation research was undertaken to better understand how individuals use photo-based social media to post about diet and weight loss. Understanding this will help public health professionals to develop more appropriate interventions and communication campaigns when using photo-based social media. The following objectives were addressed by this research: Objective 1. To systematically review dietary behavior and social media literature in order to develop a framework for studying dietary behavior on photo-based social media platforms such as Instagram. Objective 2. To collect and analyze a set of dieting for weight loss Instagram posts following the framework developed in Objective 1. Objective 3. To evaluate gender identity and seasonal trends within dieting for weight loss-related Instagram content. For objective one, a systematic review was conducted. For objectives 2 and 3, content analysis methods guided by Ideation Theory and the systematic review were used to evaluate a set of dieting for weight loss Instagram posts and to evaluate seasonal and gender identity trends. Most diet/weight loss-related social media studies focus on either Facebook or Twitter (67%), were non-experimental (41%), and were atheoretical (54%). Over half (55%) of Instagram profiles in our sample belonged to females, and most posts (57%) contained an image of food, with vegetables being the most commonly depicted food (38% of posts). Recipes and motivating language were common in the most popular posts. Females were more likely to be portrayed as thin than males, and males were more likely to be portrayed as muscular than females. Posts mentioning a specific holiday were more likely to describe the holiday as a reason for celebratory eating than a reason to restrict eating. Posts published in spring and summer were more likely to contain red meat and sugar-sweetened beverages than posts published in fall and winter. This dissertation was a preliminary step in investigating the relationship between interaction with photo-based social media and influences on dieting for weight loss. Additional research should rigorously quantify how specifically interaction with dieting/weight loss social media content is related to actual dieting for weight loss behavior.