The inclusion of a representative sample of understudied populations (e.g., women, minorities, older adults, youth, and people with disabilities) in physical activity promotion studies is a public health priority. Given the limited empirical evidence of effective recruitment strategies and limitations in research methodology for both over represented and understudied populations, the purpose of this paper was to overview the effectiveness of different recruitment techniques derived from active and/or passive approaches among mainly under represented populations. Additionally, recommendations for effective recruitment methods are proposed considering such factors as demographic characteristics and target population. Effective recruitment methodology among "all" people--regardless of age, ethnic background, functional level, or gender--is important for determining the generalizability of study findings.