Existing research has started to explore the experiences of the people around children and adults diagnosed with Asperger's Syndrome (AS) and High Functioning Autism (HFA). However, there are very few studies looking at the experiences of fathers, and there are less studies in relation to adults rather than children. This study aims to take an in depth look at the experiences of these fathers in order to start building our knowledge of the experiences of being the father of someone with a diagnosis of AS or HFA. Four fathers of sons and three fathers of daughters with a diagnosis of AS or HFA were interviewed using a semi-structured interview. The experiences of the fathers were explored qualitatively, using Interpretative Phenomenological Analysis. Analysis of the transcripts brought out four superordinate themes for the seven fathers. The first theme: 'The changing role but constant responsibility of being a "Father"' described the fathers' perceptions of their roles and responsibilities towards their child as they have grown up and into the future. The second theme: 'The importance of knowledge and understanding' described the importance the fathers placed on developing their understanding of their child and the need for other people to understand them and their child. The third theme: 'The intense emotional impact' came from the fathers' descriptions of the positive and negative emotional reactions to the challenges their children and families have faced. The final theme: 'The importance of good support' includes the fathers' good and bad experiences of support services and support from family and friends. The results of the analysis are discussed in relation to the existing literature and in terms of what they mean for the clinical practice of people working with fathers of children and adults with AS or HFA. The strengths and limitations of the study are also discussed.