For a fixed initial reference measure, we study the dependence of the escape rate on the hole for a smooth or piecewise smooth hyperbolic map. First, we prove the existence and Hölder continuity of the escape rate for systems with small holes admitting Young towers. Then we consider general holes for Anosov diffeomorphisms, without size or Markovian restrictions. We prove bounds on the upper and lower escape rates using the notion of pressure on the survivor set and show that a variational principle holds under generic conditions. However, we also show that the escape rate function forms a devil's staircase with jumps along sequences of regular holes and present examples to elucidate some of the difficulties involved in formulating a general theory.Consider a map f : M of a measure space M in which a set H ⊂ M is identified as a hole. We keep track of a point's orbit until it enters H; once this happens, it disappears and is not allowed to return.One of the first quantities of interest for such open systems is the rate of escape of mass from the system after it is initially distributed according to a fixed reference measure, such as Lebesgue measure. More precisely, given a measure µ on M, the (exponential) escape rate of µ is −ρ, whereif this limit exists. We write ρ and ρ for the lim inf and lim sup of the right hand side of (1), respectively, so that −ρ is the upper escape rate, and −ρ is the lower escape rate. Note that ρ ≤ 0, so that the escape rate is non-negative. Also, although we usually suppress the parameters, ρ depends on both the hole, H, and on the measure, µ. In the present work, µ will always be either Lebesgue measure or the Sinai, Ruelle, Bowen (SRB) measure for f , and we study the behavior of the escape rate as a function of the hole.Many works on systems with holes have focused on the existence of quasi-invariant measures with physical properties by assuming either the existence of a finite Markov partition The derivative of the escape rate in the zero-hole limit has also received attention recently [BY,KL2,FP].In this paper we consider both small and large holes and make no Markovian assumptions on the dynamics of the open systems. Let H t be a 1-parameter family of holes varying continuously with t. Let ρ(t) = ρ(H t , µ), when defined. In this context, we focus on two related questions.1. Is t → ρ(t) continuous? If so, does it possess a higher degree of regularity? 1 2. What is the overall structure of the escape rate function? Are there qualitative differences in the escape rate function as we move out of the small hole regime?The current understanding of the large hole regime is poor compared to the understanding of the small hole regime. This is because, for small holes, we may consider the open system as a perturbation of the closed system, where no mass escapes. In particular, perturbative spectral arguments have proven useful in studying this regime for certain systems, see for example [LiM,DL,KL1,DWY1]. In this paper, we extend such developments to prove the existence and Hö...