A summary of dominant defects and of positron behavior in diamonds is given. Results of the combined investigation of different types of natural and synthetic diamond with various types of defects by optical and positron annihilation (PA) spectroscopies are discussed. It is suggested that in as-grown diamonds the positron trapping is dominated by extended defects, such as impurity precipitates, multi-vacancy clusters and fluid microinclusions, whereas the trapping by nitrogen- and nickel-related point defects can be neglected.