In this chapter, I will define the sample of stars which will be discussed throughout the remainder of the dissertation. I define two coeval sets of PMS stars located in the Orion OB1 association. Once chosen, these stars can be monitored, and their rotational periods can be measured. I start with a brief description of the Orion OB1 association. I then describe the X--ray observations which were used to find the PMS stars in this association. After this, I spend several sections detailing the photometric observations and difficulties in placing objects on the H--R diagram. Once placed on the H--R diagram, the PMS objects are clear. A completeness study, however, shows that the X--ray observations do not detect all the PMS stars in the observed regions. In fact, there are a great number of very low mass objects which are detected photometricly. Some of these may have sub--stellar masses.